Silly beginner questions people would like to answer

Are tires universal? Can I put a small wheel on my car and a big tire? Can I put off road tires just because I like the way it looks on a car or will it be worse? Lets say I only drive a sedan in the city, but I like the way off road/thick tires look. Can I still put it on or will there be problems? Can I buy whatever wheel I want or are there specifics to every car?