Q: Could someone explain me, just simply, the difference between words "peg" and "dowel"?
A: In the usages which I'm familiar with, a dowel is a uniform-diameter rod, and it fits closely into its hole along all its length. Pegs fit into their holes loosely, or partly, or are tapered, or do not fit at all but just protrude to hang things on.
But in carpentry, a "pegged joint" is one which is made stronger by a perforation through both parts of the joint, into which there is put a close-fitting peg. Maybe that was originally tapered, but modern ones are straight, and they still call it a peg, not a dowel.