DuracellEnergizer said:

To put things more succinctly:

In the continuity of the Marvel comics, Boba Fett was a Mandalorian who fought in the Clone Wars alongside Fenn Shysa and other Mandalorians under the banner of the Empire before going rogue and becoming a bounty hunter.

In the post-Marvel, pre-AOTC EU, Boba Fett was a Journeyman Protector-turned-bounty hunter whose real name was Jaster Mereel. He had no known ties to the Mandalorians beyond wearing a suit of their armour he found some time after adopting his new identify.

In the post-AOTC EU, Boba Fett was once again a Mandalorian. It wasn't said so in AOTC, but the EU writers ran with the idea because -- well -- just because.

In the post-TCW continuity, Boba Fett is back to being just a bounty hunter who wears Mandalorian armour without having any ties to them.

There's also the ESB novelization, which says that Boba wears "the sort of armor" worn by Mandos.