I'm not an accountant, but "401K" defines a certain section in the IRS tax codes. This is (or could be) your retirement account. Employers will match (a portion) of what you contribute to this retirment account. Also, you can take this account with you from employer to employer (roll-over). There are many ways this can be set up, but just because an employer has a 401K doesn't mean it's a great thing.
Enron had a great plan, but it was paid on the company's own stock... totally worthless, now. Some 401K's are based or backed by weak mutual funds... large losses over the past few years. Same thing with stocks. If your employer is offering a 401K program, it is usually through an investment company (i.e. Fidelity, etc). Don't get into something that is high yield, because it will have high risk. Stay stable... contribute the maximum allowed by law... ensure your employer matches (contributes) the maximum, as well. Most importantly, make sure the 401K isn't based on company stock and make sure it is stable. If so, it CAN be a really great thing.