When you plant a potato in the ground, new shoots and plants grow from it. Planting potatoes is a form of cloning — the new plant will be genetically identical to the parent plant, and the potatoes it produces will have the same traits as the one you planted.
True potato seeds are formed in the flower when two gametes fuse; this combines traits from the mother and father plants. The flower then develops into a poisonous fruit that contains seeds.
A seed potato (a potato set for planting) gives a predictable set of traits; true potato seeds do not. That is why potatoes are commonly propagated quickly and easily from tubers.