They are, hopefully, the only part of your bike that touches the ground.
Good ones make any bike ride better, and bad ones, well, bad ones suck.
We're talking about tires. Like the bicycle itself, tires are wonderfully simple things. The tread, or knobs. The beads hold the tire to the rim. I'm not going to bore you with how we waded through knee-deep snow or welded our own frames with Bic lighters.