Tien (or Tenshinhan, whichever you prefer) is a warrior fully devoted to the martial arts craft. He is noble, very skillful, and has (in my opinion) one of the most impressive repertiores of techniques on the show.

Okay, Tien basically has two origins. I've seen a Dragon Ball movie, and in it he was a servant and bodyguard of sorts to Chaiotzu, who was an emperor. However, in the series, he is the student of one of Roshi's rivals, and faces Goku and Krillin to prove his sensei is superior. I don't know if these two origins are ever reconciled, but I suppose it doesn't matter much. Whatever his past, Tien is a perfectionist when it comes to battle... always striving to reach new peaks.

Tien faced Goku in two of the Tenkaichi Budoukai (basically a martial arts tournament). The first time, Tien won against a rather young Goku, taking the title of World Champion. The following tournament took place ten years later, and Goku defeated him in the semifinals. But despite this little rivalry, their friendship is clearly visible in the second competition. Their differences had been resolved.

Tien remains a powerful member of the Z-Senshi in Dragon Ball Z, training with Kami in preparation for the Saiyans, and after his death at the hands of Nappa, training under King Kai. Also, unlike the other Earthling Senshi, he faces Cell himself, using his Tri-Beam to hold off second-level Cell in an attempt to save Eighteen. And finally, Tien even faces Super Buu in the Buu Saga.

When Tien isn't fighting beside his allies, he's usually M.I.A., living in the frozen mountains with Chaiotzu, and always striving for self-betterment. And his training is never-ending, for his dedication has always been to the craft of martial arts.