Lost started off focusing primarily on the 324 survivors of oceanic flight 815 (referred to as the writers as "losties"), specifically the --- main characters of the story. Many of the names of the main characters were chosen due to some significants they hold, in the spirit of the show "everything is chosen for a reason". Each episode will focus on 3 or 4 of the main characters doing different things on the island but it will also pick one character specifically to focus on and show flashbacks of that characters life before the plane crash. These flashbacks relate to things that are currently happening to the character on the island and are there to help explain the way why they are how they are. In addition the flashbacks also show how each character is related to every other character on the island through 6 degrees of separation and are not even aware of it (many of the main characters are connected to one another through no more than 2 people).
*WARNING - THE FOLLOWING INFORMATION MAY CONTAIN SPOILERS IF YOU ARE NOT CAUGHT UP IN THE SHOW.
Jack Sheppard - Although no one character on the show is seen as the main character, if one had to be chosen it would be jack. Before the plane crash Jack was a prominent doctor making him the only survivor of the plane crash with any medical training (which is good for the other survivors because it seems not a day goes by where someone doesn't seem to get injured in a serious way). After helping people out of the plane wreckage Jack became the unofficial leader of the losties. He has an obsession to try and save everyone and sees it as a personal failure if someone dies under his care, even if their was nothing he could have done. He is a rational scientific person and refuses to believe in destiny or any of the other supernatural elements of the island but his character starts to change dramatically as the show progresses. Because of these leadership traits he possesses he vowed to get everyone off the island safely and sees it as his personal responsibility. The name Shepard was chosen to symbolize a Shepard leading his flock
John Locke - John Locke would be seen as the anti - Jack on the show. Instead of caring about the group as a whole he keeps to himself and focuses more on the island. Before the plane crash John was in a wheelchair unable to walk but after the plane crash he was able to again. He believes in destiny and the supernatural, which gets him into many disagreements with Jack. Like Jack however he also goes through a character change and starts to lose his faith, he also begins to see himself as a leader and cares more about his people. John has a communion with the island and seems to understand why things happen before they do (he has the uncanny ability to know when it is going to rain when there are no signs of it). The name John Locke was chosen after the famous philosopher of the same name due to both the character and philosophers insights (in the later season he went by an alias of Jeremy Benthem, also a philosopher who followed the works of John Locke).
Hugo "Hurley" Reyes - Hugo acts as the comic relief in the show, at least in episodes that don't focus on him. Other characters refer to him as Hurly, a nickname that was given to him before he arrived at the island, although he will not tell anyone where it came from. While on the island he tries to make all the other survivors more comfortable about their situation, in one case he made a golf course that is one hole long using materials from the plane and some golf clubs that a passenger was transporting. The only counter to his up attitude is shown during his flashback episodes where it reveals his past is filled with bad luck and mental instability.
Charlie Pace - Charlie is a washed up musician from the band DriveShaft, which had a one hit wonder called "You All Everybody". Charlie had an addiction to heroine which was a big focus near the beginning of the show as he went through detox. Charlie had a love interest of Claire, a single pregnant mom who was also on the plane.
Claire Littleton - Claire was a pregnant woman who was on her way to the states to give her baby up for adoption when the plane crashed, miraculously her and the baby were unharmed. It is later revealed through flashbacks that her father and Jacks father are the same person, which makes them half - siblings, although none of them discover this the entire time they know each other on the island.
Michael and Walt Dawson - Walt is a 10 year old boy who lived with his mom for most of his life until she died and he went to live with his father Michael. The two of them seem to get into many disagreements on the island due to the fact that Walt believed his father abandoned him and Michael is not used to being a father. Many strange occurrences happen around Walt which the show has yet to explain, but the writers have said that they tie in with the big picture storyline.
Jin and Sun Kwon - Two Chinese citizens who keep to themselves due to the language barrier. It is later revealed that Sun can actually speak English but is too afraid of her husband to reveal it. She learned it when she was planning on running away from her husband and father, both of which are very powerful men and would do anything to track her down.
James "Sawyer" Ford - James Ford is a con man who sees this plane crash as more of an opportunity than a tragedy. While everyone else is still in shock about the recent incident James starts scavenging luggage and possessions off dead bodies to barter with, taking full advantage of the situation. It is later revealed through flashbacks that as a child his parents were killed due to the actions of a con man and that drove him to become one himself as he searched for the man to exact his revenge. The only thing he knows about the man is the alias he used at the time which was Sawyer, he took the name for himself, the only people who call him by his real name are Kate and Locke (who calls everyone by their real name). The Name Sawyer was chosen by the writers in reference to Tom Sawyer, the famous con man who tricked others into doing his job for him.
Kate Austen - Kate serves as the primary love interest in the show. She is involved in a love triangle between Sawyer and Jack, as she has expressed interest in both of them. Kate starts off seeming innocent but it is quickly revealed in the show that she was on the plane as a convict that was being extradited back to the states for murder. In spite of this she comes off as very caring and always puts herself in dangerous situations for the good of the group.
Sayid Jarrah - Sayid was going to the states to meet with his childhood love Nadia when the plane went down. It is later revealed through flashbacks that many of the other characters have had some contact with Nadia in the past before the plane crash. Sayid worked for the national guard as an "interrogator" in which he would use torture to extract information, his skill set has come up on a few occasions in the coarse of the show.
Rose Nadler - Rose is a terminally ill woman who was traveling with her husband Bernard when the plane crashed. Even though everyone else believes that the passengers from the tail end are dead Rose is adamant that her husband is still alive (which it turns out he was). After she arrives on the island for unexplained reasons her cancer is cured so she opts to stay on the island with her husband, even if rescue were to come.