Park Tool and Sheldon Brown web sites are great sources of (free) information about how most things work. I'll also give myself the liberty to recommend my site, BikeGremlin, with most standards and parts compatibilities (mix-maching) explained.

Derailleurs and gear hubs have their good and weak points each. In a car like setup, a gear hub would require some kind of chain tensioner, if rear to front chainring distance is fixed. If you find a way to move one of those relative to the other and lock it in place, then a chain tensioner is not necessary.

Dearailleur pros: no slipping under load, cassettes often provide a wider gearing range than IG hubs, a bit lower total weight (RD + cassette vs IG hub).
IG hub pros: allows shifting gears when stationary - good for a car like system. Works in deep freeze environment, doesn't freeze up from snow.