401(k)s and similar plans - 403(b)s, 457s and Thrift Savings Plans - are ways to save for your retirement that your employer provides, or "sponsors." You may hear people describe them as "defined contribution plans." That name comes from the fact that you make contributions to the plans - that is, you put your own money into them. (You may also hear your employer describe the plan simply as "The Company X Savings Plan.")
Here's how they break down:
401(k) plans are the most common type of defined contribution plan, so they're what you may read and hear about most often. But in fact there are no huge differences between a 401(k) plan and the other defined contribution plans (beyond who can use them, of course).