Deidara was a rogue ninja from the Hidden Stone Village who was the former pupil of the 3rd Tsuchikage. Despite often annoying his sensei, Onoki was immensely proud of Deidara's skills. During his days as a shinobi of the Stone Village, Deidara was a member of the Explosion Corps as he possesses the Explosion Style kekkai genkai which give the wielder the ability to utilise explosive chakra techniques in combat.
Deidara had a keen interest in creating clay sculptures and craved a special forbidden jutsu of the Stone Village that allows a person to manipulate their chakra into substances. Stealing the technique and fleeing his village, Deidara became a rogue ninja who utilised his new skills by becoming a terrorist-for-hire. This is how the Akatsuki discovered him and after a short confrontation with Itachi Uchiha, Deidara accepted their offer and joined the organisation.
"Their not weird, their perfect!"