Acquaintance: not a true friend—sharing of emotional ties are absent. An example would be a coworker with whom you enjoy eating lunch or having coffee, but would not look to for emotional support. Many "friends" that appear on social networking sites are generally acquaintances in real life.
Best friend (or the closest friend): A person with whom someone shares extremely strong interpersonal ties with as a friend.
BFF ("best friend forever"): Slang used primarily in the USA by teenage and young adult women to describe a girl friend or close best friend.
Blood brother or blood sister: Either people related by birth, or a circle of friends who swear loyalty by mingling the blood of each member together though not recommended for risk of blood disease.
Boston marriage: An antiquated American term used during the 19th and 20th centuries to denote two women who lived together in the same household independent of male support. Relationships were not necessarily sexual. It was used to quell fears of lesbians after World War I.
Bro: Slang used primarily in the USA, Australia and New Zealand by teenage and young adult men to describe a boy friend or close best friend. This term is currently used to describe the modern generation of college-age male party-goers. The name is typically associated with attention-seeking males who like to get drunk and party constantly.
Sis: Also slang used primarily in the USA like "Bro" but for women and girls.
Buddy: In the USA, males and sometimes females often refer to each other as "buddies", for example, introducing a male friend as their "buddy", or a circle of male friends as "buddies". Buddies are also acquaintances that you have during certain events. The term may also refer to an online contact, such as the AOL Buddy List.
Casual relationship or "friends with benefits": A sexual or near-sexual and emotional relationship between two people who don't expect or demand to share a formal romantic relationship. This can also refer to a "hook-up".
Family friend: A friendship extended to family members of the friends. Close relation is developed in those societies where family setup is strong. This term usually used in subcontinent.
Comrade: Means "ally", "friend", or "colleague" in a military or political connotation. This is the feeling of affinity that draws people together in time of war or when people have a mutual enemy or even a common goal. Friendship can be mistaken for comradeship. Former New York Times war correspondent Chris Hedges wrote:
We feel in wartime comradeship. We confuse this with friendship, with love. There are those, who will insist that the comradeship of war is love – the exotic glow that makes us in war feel as one people, one entity, is real, but this is part of war's intoxication. [...] Friends are predetermined; friendship takes place between men and women who possess an intellectual and emotional affinity for each other. But comradeship – that ecstatic bliss that comes with belonging to the crowd in wartime – is within our reach. We can all have comrades.[8]
As a war ends, or a common enemy recedes, many comrades return to being strangers, who lack friendship and have little in common. Sometimes they even become enemies in another war.
Cross-sex friendship: A person having a friend of the opposite sex with having little or no sexual or romantic activity: a male who has a female friend, or a female who has a male friend. Historically cross-sex friendships have been rare. This is because often men would labor in order to support themselves and their family, while women stayed at home and took care of the housework and children. The lack of contact led to men forming friendships exclusively with their colleagues, and women forming friendships with other stay-at-home mothers. However, as women attended schools more and as their presence in the workplace increased, the segregated friendship dynamic was altered, and cross-sex friendships began to increase. Cross-sex friendship has once been a sign of gender deviance, but now it has been loosened because of the increase of gender equality in schools and the workplace, along with certain interests and pastimes such as sports.
However, cross-sex friendships aren't always a socially accepted norm of amity and some of those friendships could develop into romantic feelings (see romantic friendship). However, when these feelings are not mutual, they can often backfire, making it hard for the two to remain friends.
Frenemy: A portmanteau of the words fr(iend) and enemy, the term frenemy refers to someone who pretends to be a friend but actually is an enemy—a proverbial wolf in sheep's clothing in the world of friendships. This is also known as a love–hate relationship. Most people have encountered a frenemy at one time or another in the same places one might find friends—school, work, the neighborhood. The term frenemy was reportedly coined by a sister of author and journalist Jessica Mitford in 1977, and popularized more than twenty years later on the third season of Sex and the City. While most research on friendship and health has focused on the positive relationship between the two, a frenemy is a potential source of irritation and stress. One study by psychologist Dr. Julianne Holt-Lunstad found that unpredictable love–hate relationships characterized by ambivalence can lead to elevations in blood pressure. In a previous study, the same researcher found that blood pressure is higher around friends for whom they have mixed feelings than it is when they're around people whom they clearly dislike.[9]
Fruit flies,[10] fag hag (female),[11] or fag stag (male)[12]: denotes a person (usually heterosexual) who forms deep ties or close friendships with gay men. Men (gay or straight) who have lesbian friends have been referred to as lezbros or lesbros.[13] The term has often been claimed by these straight members in gay-straight friendships, however some feel that it is derogatory.[14][15]
Imaginary friend: a non-physical friend created by a child or even an adult. Sometimes they're human, other times they're animals like the life-size rabbit in the old Jimmy Stewart movie, Harvey. Imaginary friends are also created for people desperate for social interaction but are isolated from contact with humans and pets. It may be seen as bad behavior or even taboo (some religious parents even consider their child to be possessed by an evil "spirit"), but is most commonly regarded as harmless, typical childhood behavior. The friend may or may not be human, and commonly serves a protective purpose.
Internet relationship: a form of friendship or romance which takes place over the Internet. Some internet friendships evolve into real life friendships. Internet friendships are in similar context to a pen pal. These friendships are also based on the thought that the other person that they may not have ever met in real life knows them for who they are instead of the mask they may use in real life.
Mate: In the UK, Ireland, Australia, and New Zealand, blokes often refer to each other as "mates", for example, introducing a male friend as their "mate", or a circle of male friends as "mates". In the UK, as well as Australia, this term has begun to be taken up by women as well as men.
Open relationship: a relationship, usually between two people, that agree each partner is free to have sexual intercourse with others outside the relationship. When this agreement is made between a married couple, it's called an "open marriage".
Communal Friendships: A friendship where the friends gather often to provide encouragement and emotional support in times of great need. this type of friendship tends to last only when opposing parties fulfill the expectations of support for the relationship.[16]
Agentic Friendships: A friendship where both parties look toward each other for help in achieving practical goals in their personal and professional life.[17] These friends help with completing projects, study for and exam, or help a friend move out. These types of friends value sharing time together, but only if there are no other priorities and the friend is actually available to help in the first place. Emotions and sharing of personal information is of no concern to this friend type.
Pen pal: people who have a relationship via postal correspondence. Now pen pals has been established into internet friendship with the use of chat or social networking sites. They may or may not have met each other in person and may share either love, friendship, or simply an acquaintance between each other. This type of correspondence was encouraged in many elementary school children; it was thought that an outside source of information or a different person's experience would help the child become more worldly.
[edit] Friendship and health
The conventional wisdom is that good friendships enhance an individual's sense of happiness and overall well-being. But a number of solid studies support the notion that strong social supports improve a woman's prospects for good health and longevity. Conversely, it has been shown that loneliness and lack of social supports are linked to an increased risk of heart disease, viral infections, and cancer as well as higher mortality rates. Two female researchers have even termed friendship networks a "behavioral vaccine" that protects both physical and mental health.[18]
While there is an impressive body of research linking friendship and health status, the precise reasons for this connection are still far from clear. Most of the studies are large prospective studies (that follow people over a period of time) and while there may be a correlation between the two variables (friendship and health status), researchers still don't know if there is a cause-and-effect relationship, e.g. that good friendships actually improve health.
There are a number of theories that attempt to explain the link, including that: 1) Good friends encourage their friends to lead more healthy lifestyles; 2) Good friends encourage their friends to seek help and access services, when needed; 3) Good friends enhance their friends' coping skills in dealing with illness and other health problems; and/or 4) Good friends actually affect physiological pathways that are protective of health.
source:http://en.wikipedia.org/wiki/Friendship 
