A 401K is a retirment savings plan that is established through your employer. Money is taken out of your paycheck (pre-tax) and put into your 401K account (FYI, The "401K" comes from the section of the U.S. Tax code that covers how these accounts are managed for tax purposes.) In most cases you are given a list of several investment options (usually mostly mutal funds) that you can direct that the funds in your account be invested in.
If your employer has a defined benefit retirement plan than you probbaly won't have the option of a 401K. In most cases you only have one retirement vehicle through an employer. They don't offer competing plans.
The Motely Fool WWW site has a lot of good info on 401Ks (and other investment options too!)
http://www.fool.com/money/401k/401k.htm
(Your RRSP appears to be very similar to what we would call an "IRA" in the U.S. btw!)