Pension refers to an annuity (A right to receive amounts of money regularly over a certain fixed period) paid to a retired employee as a benefit. Pension refers to the money you receive as a benefit after you have retired from your place of work. There are different kinds of pension schemes. These pension schemes are usually set up by the government, employers, and insurance companies.
At times, there are some people who receive a disability pension. This type of pension is for people that can't work any longer due to cases like accidents or diseases. 401k, on the other hand, can be considered as a saving account opened by an employee in a bid to save for retirement.
Those with 401k accounts will not be paying taxes on their savings. However, these taxes will be deducted when the savings are withdrawn. While a pension is paid regularly over a certain period of time, the amount you get from 401k depends on your savings.