A Sith lord, Darth Sidious was the prime practitioner of "The Dark Side of the Force". In The Phantom Menace, Sidious sent out his apprentice Darth Maul to kill Queen Padme Amidala and the Jedi knights Qui-Gon Jinn and Obi-Wan Kenobi.
Later, supposedly in Star Wars Episode 3, it is generally assumed by fans that Sidious will be revealed to be actually Supreme Chancellor Palpatine, and take on Anakin Skywalker as his new apprentice under the name Darth Vader. If so, then between Episode III and Episode IV, Darth Sidious will become the Emperor who is seen in Episodes V and VI.
There are clear clues to this fact earlier, for example in the closing scenes of The Phantom Menace, Palpatine is shown in profile, matching Sidious seen in the same position but with his face hidden by the hood of his cloak. Both Darth Sidious and Palpatine are also played by the same actor, Ian McDiarmid.