(1) Leibniz: "We are now, finally, ready to get a picture of what Leibniz thinks the universe is really like. It is a strange, and strangely compelling, place. Around the end of the Seventeenth Century, Leibniz famously began to use the word "monad" as his name for substance. "Monad" means that which is one, has no parts and is therefore indivisible. These are the fundamental existing things, according to Leibniz. His theory of monads is meant to be a superior alternative to the theory of atoms that was becoming popular in natural philosophy at the time. Leibniz has many reasons for distinguishing monads from atoms. The easiest to understand is perhaps that while atoms are meant to be the smallest unit of extension out of which all larger extended things are built, monads are non-extended (recall that space is an illusion on Leibniz's view)."
(2) Cateogry Theory and Haskell: "Monads are obviously an extremely important concept in Haskell, and in fact they originally came from category theory. A monad is a special type of functor, from a category to that same category, that supports some additional structure."
(3) A monad is like a burrito.