From my understanding, a pension plan is provided by the company and pays each employee who retires so much money for life. My grandpa retired from GM back in I think the 80s, and he's still getting money from his pension. A 401K is like an account that employees put their own money into and manage. Sometimes the company will contribute some as well. If not managed well, the money can run out during retirement and force people back to work. Also sometimes people live longer than they originally expected to.

https://www.investopedia.com/ask/answers/100314/whats-difference-between-401k-and-pension-plan.asp