Darth Sidious found young Maul before the Jedi did and raised him to be a fierce Sith Lord. He was Sidious' apprentice while Sidious was apprenticed to Plagueis. He was trained harshly by Sidious and was eager to fight the Jedi.
In Episode I, Sidious had Maul track Queen Amidala of Naboo to Tatooine. There, he found the Jedi Qui-Gon Jinn and attacked him before Qui-Gon to get to his ship. But Maul was unable to defeat the Jedi, who escaped on the ship.
When Qui-Gon and his apprentice, Obi-Wan Kenobi, return with the queen to Naboo, Darth Maul awaits them. The Jedi fight Maul while the queen and her soldiers enter the palace. The two Jedi fight fiercely against Maul, but neither of them land a blow on the Sith Lord. The three are momentarily separated by lasers, and Qui-Gon resumes the duel before his apprentice. Before Obi-Wan can reach the battle, Maul. stabs the Jedi Master and Qui-Gon falls to the floor.
Darth Maul waits for the laser to let the apprentice in, and quickly begins to fight him. He knocks Obi-Wan into a hole and the Jedi looses his lightsaber. Obi-Wan calls his fallen master's lightsaber to him with the force and leaps out of the pit. With a quick swip, the apprentice cuts the Sith Lord in half at the waist. Darth Maul tumbles down the shaft.
But Maul survives the fall and returns in the Clone Wars. . .
I think that Darth Maul is my favourite villian in the Star Wars movies (Don't ask why, he just is.)