All concrete cracks eventually. Doesn't mean it's going to break apart - there's steel mesh and/or rebar inside. Usually it's caused by ground settling. There are crack fillers sold at Lowes and the like. Frankly, they look like hit. The color stands out, and of course the stuff's too thick to flow deep in the crack where you could smooth it out. It ends up looking like the street after the city puts tar on the cracks in it.
Hairline cracks, I'd leave alone. If your expansion joints are crumbling, do something about that, because that's where water gets beneath the concrete. Sometimes it's redwood bender board that has rotted over the years, sometimes it's that plastic Crud that gets brittle with sunlight and breaks up. There are some nice caulk-like products that you can use to seal those areas up.