Leucippus was one of the Pre-Socratic philosophers who developed the atomist theory, which explained that all matter is made up of indivisible particles. (The word atom means 'not cut'.) He is generally considered its founder, but Leucippus worked with Democritus of Abdera as either his colleague or teacher, so the atomist theory may have been a collaboration. Zeno of Elea may have taught Leucippus.
Leucippus was born in Abdera, Elea, or Miletus in the 5th Century B.C., possibly living from c. 480 - c. 420 B.C. We don't know more than that, although Epicurus denied there even was a Leucippus. He is thought to have come up with the idea that the universe contains the solid (made up of atoms) and the void. Together they make up everything in the universe. Aristotle says he was trying to develop a theory that went along with the evidence of our senses.
Leucippus is thought to have said that nothing happens in vain but that everything happens from logos (translated in Biblical contexts as "The Word") and necessity.
Source: Berryman, Sylvia, "Leucippus", The Stanford Encyclopedia of Philosophy (Fall 2004 Edition), Edward N. Zalta (ed.).