Monica is a naturally competitve, neat-freak of the group. Monica aquired the big purple apartment, where many memorable moments of the show take place, from her grandmother a couple of years before the show began. Monica's childhood wasn't a great one (even though it seems like cake compared to, say, Phoebe or Chandler's childhoods). She and her mother, Judy, never got along well. Her father, Jack, seemed to like her brother Ross better then her and so Monica spent most of her time eating. She lost all the weight because Chandler called her fat one Thanksgiving before the show even started and she's now increadably thin. The others consider her to be very bossy and controlling.
She is probably the most important of the group, in Chandler's mind anyway. The two were always close, though it seems that Monica just thought of him as her 'annoying friend, Chandler'. That is, until that night in London where their romance began. The two dated for years, until they were married on May 15th, 2001. You can find out more about Chandler and Monica's relationship here.
joey tribbiani
Joey seems to be Chandler's best male friend. The two lived together for years. Joey works as an actor, though he hasn't really done anything promising. Many of the deeper things the group talks about go over Joey's head, as he isn't extremely intellectual. He is quite active with the females, but never has a lasting relationship on the show.
Chandler and Joey are extremely close. They have a lot of fun together which seems to make Chandler forget about all of the money Joey owes him. Joey's favorite show is Baywatch (just like Chandler's) and they often watched it together back when they lived in the same apartment. Chandler seemed to resent Joey just a little, probably because Chandler has problems with women and Joey really, really doesn't.
phoebe buffay
Phoebe is the 'crazy one' of the group, often the maker of strange comments. She appears a little ditzy and flaky, but that's just her. Phoebe left home early in life after her mother committed suicide. She moved to the city and met "an albino guy" who became her roommate and then he committed suicide. Aroma-therapy supposedly saved Phoebe's life. She appears to be very spirtual; cleansing auras and she once thought that a random stray-cat held the spirit of her mother. Phoebe has an extremely complicated family-life (she doesn't get along with her twin sister Ursula) but she seems to really love her family (an example of this would be the 'favor' she did for Frank Jr and Alice).
I don't think that it could be more obvious that Phoebe dislikes Chandler, at least a little. The two appeared to be good friends throughout the first four seasons (though Phoebe did make a few little comments here and there) until Chandler and Monica started dating. Phoebe seems sure that there is someone out there who is better for Monica then Chandler and she constantly makes this obvious. Despite all of this, Chandler and Phoebe still remain friends.
rachel green
Rachel made quite an enterance into our lives (and the group's) in a wet wedding-dress, after running out on her would-be husband Barry. She is the product of a very well to-do family and this was apparent at the beginning of the series, when Rachel was cut off from all her father's money after her run-away bride episode. She was a waitress at Central Perk for a long time, despite the fact that she hated the job and was awful at it. Rachel shared a long, promising relationship with Ross, but (sadly) the relationship ended with Ross cheating on her. Rachel was also the first of the group to have a baby, Emma Geller-Green.
Chandler and Rachel aren't extremely close. She does like him enough to let him play match-maker for her, several times. He also helped her through her parent's divorce and was extremely depressed when she and Ross broke off their relationship. Rachel, like the other girls, enjoys giving Chandler relationship advice.
ross geller
Ross has a Ph.D. in paleontology and used to work for a museum. His first wife, Carol, divorced him after she realised/decided that she was a lesbian. Ross speaks very precisely and is sometimes shy. He spent much of the first season with his pet monkey, Marcel. He dated Rachel but ended up cheating on her and they broke up. Ross seems to be big on divorce, he's had three and he's not happy about it at all though the group jokes about it all the time.
Ross and Chandler roomed together in college and Ross often brought Chandler home with him for Thanksgiving and other such holidays (Chandler met Monica at these family get-togethers, as well). At Ross's first wedding to Carol, Chandler was the best man. Chandler was again Ross's best man (well, one of them) at his wedding to Emily in season 4. Ross is Chandler's other best friend.