401(K) plans are tax-deferred retirement savings plans for employees. The employer sets them up, and each company has a slightly different 401(k). They are part of a family of retirement plans known as "defined contribution" plans—the amount contributed is defined by the employer or the employee.
When you join a 401(K) plan, you tell your employer how much money you want to contribute to your account. This amount is deducted from your salary before taxes are applied, so you pay less income tax. More importantly, the money is deducted even before you have received it, making it the easiest savings plan to contribute to. Your employer may match a portion of your contribution.
The money is invested by the plan administrator (on your behalf) in mutual funds, bonds, money market accounts, etc. You decide the mix of investments. They usually have a list of investment vehicles you can choose from as well as some guidelines for the level of risk you are willing to take. Since the plan is an incentive for retirement savings, there is one condition: if you withdraw the money before you are 59½ years old, you will have to pay tax as well as a 10% penalty fine to the IRS.