Contracts are generally only for contracted (i.e. non-employee( work. A self-employed consultant, programmer, designer, etc. will have a contract definining the terms of the engagement.

Employment is "at-will" in most states. Employers are obligated to pay for work performed, but you can be terminated at any time (and you can quit at any time).

Some high-level jobs will also have a written contract but it's not normal for a typical salaried or hourly position, unless there is a union involved.