The Untyped Lambda Calculus: Where It All Begins
Details
Tikhon Jelvis presents the untyped lambda calculus.
The rules describing the untyped lambda calculus are absurdly simple. But those rules not only lead to a rich and fascinating field of study, they are also the metaphor in which we nearly always think about computation. You already use the lambda calculus every day, even if you don't know it.