An Assassin or Hitman is a person (a vast majority of the time a gangster) that is payed to kill people. They play a considerable role over the course of the GTA games, and the player is usually an assassin themself.
Claude, protagonist of Grand Theft Auto III, was formerly a bank robber (among other crimes) before his girlfriend Catalina turned on him and left him for dead, if not the cops. He then became an associate of the Leone Family but was later chased out of town for suspicion of having an affair with the Don's wife. He then became an associate of the Yakuza, which included assassination. Then he started working for Ray Machowski, the Yakuza mole in the LCPD. Claude became an assassin for Machowski as well. He later became an assassin for Donald Love, friend of Machowski.
Tommy Vercetti was the personal assassin for Sonny Forelli, Don of the Forelli Family 15 years prior to the events in Grand Theft Auto: Vice City. Don Forelli eventually hired other assasssins to kill Vercetti himself (for motives unknown) but they were all killed by their target. Vercetti earned the nickname "The Harwood Butcher" and spent 15 years in prison. When he got out, he was transferred to Vice City, eventually started the Vercetti Gang, and ended up killing Don Sonny Forelli for his betrayal. Notably, he took the place of Leo Teal in Mr. Black's Assassination Ring.
After CJ's brother Sweet was sent to prison, he started working for Frank Tenpenny as an assasissin. Tenpenny was a corrupt cop and CJ killed people that would expose him and his partner Eddie Pulaski. CJ went on to be an assassin for the Leone Family and Triads.
Being a member of the Leone Family, being an assassin is part of the job. Prior to the events in Grand Theft Auto: Liberty City Stories, Toni was forced into leaving Liberty City after killing a rival made man for Don Salvatore Leone. Upon coming back, he resumed his work. Toni worked as an assassin also for Donald Love.
After being dishonorably discharged from the US Army, Vic started working for Phil Cassidy, Marty Jay Williams, Umberto Robina as an assassin and also starting his own gang. It seems he would always be assassinating Cholos and Bikers. Later in the game, he would chiefly assassinate rival Drug Lords under orders from the Mendez Brothers or just for his own power.
Niko came from Europe to seek "The American Dream" and started working for the Russian Mafia, and later the Italian Mafia (notably the Pegorino Family and Gambetti Family). He also worked for the Irish and Little Jacob. During his time in Liberty City he also contacted the enigmatic Fixer, who sent Niko on assassination missions specifically.
Ray Bulgarin had Luis Lopez kill some people for him including the owner of a sports team. Luis had told Bulgarin that he is no one's hitman or in that type of business, however Bulgarin forced Luis to do the jobs anyway.
Huang's father was son of the Triad Leader, and several of his jobs required assassination.
Leo Teal was a chef in Ocean Beach, Vice City, but also a Drug Dealer and assassin for the mysterious "Mr. Black". Teal, as hinted by several phone calls, played a major part in the ambush of the Forelli-Vance Deal. After selling some of the drugs to Ricardo Diaz, he was killed by Tommy Vercetti when he wouldn't reveal any information.
Several unnamed assassins were emloyed over the course of the games. Ma Cipriani employed an army of assassins to kill her own son. They wore completely black suits and were bald with 5:00 shadow. Vincenzo Cilli employed Leone assassins to kill Toni as well, dressed as casual Leones (black dress suites with gloves, black goaties and slick hair). Normally, mafia assassins dress as their own gangsters. One assassin killed Roman Bellic at his wedding in an attempt on Niko's life, but was killed by Niko shortly after. He wore a plain black suit and was bald.