In general, a pension is an arrangement to provide people with an income when they are no longer earning a regular income from employment. It is a tax deferred savings vehicle that allows for the tax-deferred accumulation of a fund for later use as a retirement income. Pensions should not be confused with severance packages; the former is paid in regular installments, while the latter is paid in one lump sum.
A traditional defined benefit (DB) plan is a plan in which the benefit on retirement is determined by a set formula, rather than depending on investment returns. In the US, 26 U.S.C. § 414(j) specifies a defined benefit plan to be any pension plan that is not a defined contribution plan where a defined contribution plan is any plan with individual accounts. A traditional pension plan that defines a benefit for an employee on that employee's retirement is a defined benefit plan.
In a defined contribution plan, contributions are paid into an individual account for each member. The contributions are invested, for example in the stock market, and the returns on the investment (which may be positive or negative) are credited to the individual's account. On retirement, the member's account is used to provide retirement benefits, sometimes through the purchase of an annuity which then provides a regular income. Defined contribution plans have become widespread all over the world in recent years, and are now the dominant form of plan in the private sector in many countries.