Gotenks, Vegito, Gogeta, Android 18, Uub, Fat Uub, Nail, Android 16, Master Roshi, Kami, Pikkon, Nuova Shenron, Pan

The Z Fighters are a small group of super warriors on Earth. Their duty is to defend Earth from major threats and to protect the seven mystical Dragon Balls from the hands of evil. The Z fighters are not organized in any way, because it is simply a group of super fighters who all share the same interest of protecting their home.
The first actual formation of the Z Team was out of necessity when Goku was faced with Raditz, who had kidnapped his son Gohan and wanted to destroy the Earth. Goku formed an alliance with his arch rival Piccolo in order to fight for a common cause. The alliance made from necessity was the birth of the Z team.
The term itself is very rarely actually used in the series, the first, and one of the only occurrences being in the androids saga, when Trunks explains to Goku the order in which "The Z fighters" die, where it becomes something of a plot hole or inconsistency, as the term seemed to have come out of nowhere. The characters involved wouldn't really consider themselves a 'team'. The group works as a team and help each other, but it is limited to the aspect of helping one another simply because of friendship. The term most commonly used when the characters refer to themselves is the Earth's Special Forces in the FUNimation dub, or just simply "Senshi" (Warriors/Fighters) in the original Japanese.
The Z Fighters were also in Dragon Ball, although they were called the Turtle Hermit School as they consisted of the students of Master Roshi, although Tenshinhan, Chiaotzu, and Yajirobe eventually "join" even though they never become Turtle Hermit School students. They all consist of humans at this point (Goku is thought to be a "human" in Dragon Ball).
The term is first used to describe the main characters during the Jinzō'ningen Saga, though it is generally meant by fans to indicate the rough teams from the beginning of Dragon Ball Z.
It wasn't really until the Android saga that the team of the Z Fighters came full circle. Goku returns from outer space one year after defeating Frieza, while Trunks comes from the future to warn everyone about the Red Ribbon Androids that have caused mayhem in his time. (See: Android Saga) Once Vegeta decides to train against the Androids, and Krillin, Tenshinhan, Chiaotzu and Yamcha came back to life, it is around this time that the series refers to the fighters as the Z-Senshi. Trunks also returns to complete the group. With Goku, Gohan, Piccolo, Trunks, Krillin, Tien Shinhan, Chiaotzu, Yamcha and Vegeta the group fully forms and it is shown through the opening sequence at this time in the series. (The final image is of the aforementioned fighters and Goku as he transforms into a Super Saiyan.)
It is also worth noting that while this version the the Z Fighters is a prominent one during the series history, it is the only period of time in which the team consists of these certain members. The Android/Cell Saga was a turning point in the series for a number of reasons, one of them being that the roster of fighters was changed after the Cell Games. By the end of the Cell Games, Goku is dead and Gohan has become Earth's primary defender. Trunks returns to his timeline more powerful than ever, thanks to his training in the Room of Spirit and Time with Vegeta, and proceeds to finally destroy the evil versions of Androids #17 and #18 in his world, as well as his timeline's Cell awhile later. Meanwhile, Tien bids farewell to the Z Fighters and plans to live in seclusion with his friend Chiaotzu, having used all his strength to help fight the Androids and protect the Earth. Yamcha retires from fighting and never trains again. Krillin continues to live with Master Roshi, and after an uncertain amount of time eventually hooks up with #18 and gets married. Piccolo decides to live with Mr. Popo and Dende on Kami Tower, partly because there is no more reason to train, partly because his pupil Gohan has finally surpassed him, and partly because part of him is still Kami-Sama and wishes to continue to reside over the Earth. Vegeta continues to train, hoping to become a Super Saiyan 2 like Gohan, but declares that he will never fight again with his only rival, Goku, dead. Finally, Gohan keeps his promise to his mother Chi Chi and gives up fighting to study and become a scholar.
By the beginning of the Buu Saga seven years later, the Z Fighters consists of many "next-gen" type of characters. Gohan is eighteen and is the primary hero of Earth, both as a Super Saiyan and his alter ego "Great Saiyaman". The Z-Team isn't really continued, per se, based on the fact that their opponent Majin Buu was so super powerful, it mainly called upon one single person to fight him. However, considering the fighters in this time period, it can be said that the "New Z Fighters" consists of the following: Goku, Gohan, Piccolo, Vegeta, Trunks, Goten, Krillin, Videl, and Android 18.
Also worth noting is that unlike the Android Saga, the Buu Saga is when Vegeta voluntarily became a "good guy". He fought for his family, and to save the Earth. He also came up with the idea of using a super-powered "Chou Genki-Dama" (Super Spirit Bomb) that defeated Buu at the very end.
Finally, for Dragon Ball GT, the main fighters tend to be Goku, Trunks and Gohan's daughter, Pan, for the most part. Uub does fight in the first major arc, the Baby Saga. However, the "Z Fighters" (Or "GT Fighters") do come together in the Super 17 saga. While Goku's trapped in Hell, Android 17 kills Krillin. This forces the hand of the last major, briefly retired fighters from the end of Dragon Ball Z. The following are the fighters who fought against 17: Vegeta, Trunks, Goten, Gohan, and Uub.
The actual members of the "team" consists of two at the beginning of Dragon Ball Z and grows to about ten by the end. Even though it is for most of the series, reduced to the Earth's welfare, the members are a mixing pot of humans and alien races such as the super warrior beings called Saiyans. Saiyans are the primary fighters on the team due to their extreme power and war-like tendencies.