"the relation between an employer and an isolated employee or worker is typically a relation between a bearer of power and one who is not a bearer of power. In its inception it is an act of submission, in its operation it is a condition of subordination, however much the submission and the subordination may be concealed by the indispensable figment of the legal mind known as the 'contract of employment'. The main object of labour law has been, and I venture to say will always be a countervailing force to counteract the inequality of bargaining power which is inherent and must be inherent in the employment relationship.
In Roman law the equivalent dichotomy was that between locatio conductio operarum and locatio conductio operis (lit. hire of services and of service).
The terminology is complicated by the use of many other sorts of contracts involving one person doing work for another. Instead of being considered an "employee", the individual could be considered a "worker" (which could mean less employment legislation protection) or as having an "employment relationship" (which could mean protection somewhere in between) or a "professional" or a "dependent entrepreneur", and so on. Different countries will take more or less sophisticated, or complicated approaches to the question.
Normally, such contracts provide for termination of employment, by either party, and include associated matters such as notice period, compensation arrangements and, sometimes, garden leave.
Some employers use non-disclosure and non-compete clauses to protect their trade secrets from being dispersed when employees leave. Depending on where people live, the laws regarding enforceability of these clauses vary widely.
UK law holds that employment contracts have implied terms (assumed, unspoken, essential terms ), as well as explicit terms (typically those in writing). Legal precedent provides for example that there is an implied contractual term of trust and confidence, meaning each party to the contract is expected to behave in a manner allowing the other to maintain trust and confidence in the other.