Each plan is either a defined contribution plan or a defined benefit plan. A defined contribution plan is a bucket of money with the employee's name on it. It involves a stated dollar contribution made to the plan periodically, either by the employer, the employee, or both. Defined contribution plans have become increasingly common as defined benefit plans have become less common. The balance in a defined contribution plan is constantly changing, but its value is ascertainable at any given point.
Most employees receive an updated statement at least once a year.
A defined benefit plan works differently. It's not a bucket of money. There's no stated dollar contribution to it for each employee. Instead, the employer makes a promise to pay a benefit to the employee at some point in the future. Usually, the amount promised is a function of some kind of formula, often based on the employee's pay rate near the end of his or her career and the number of years the employee worked before retirement.