what are the essentialia of an employment contract

Yes, an employee can cancel employment as long as there was no contract for that employment. If there was a contract, the employee can be sued for breech of contract.

A contract of employment is a document which describes the employer and employee agreement. The document contains the duties to be done over a certain duration and the amount of remuneration to be expected.

Yes, Title VII of the Civil Rights Act can override an employment contract if the terms in that contract conflict with the law. The contract cannot overrule because Title VII is codified law (is that the right term?) and therefore supersedes an employment contract.

An employment contract dictates the conditions of employment, such as salary, vacation, benefits, etc. An "at will" employee serves at the pleasure of the employer, meaning their employment can be terminated at any time, for any reason, or for no reason at all. The effect of an employment contract on an at-will employee would be to set salary, benefits and so on as long as the employee worked at that employer.