well obviously, if you're not putting any in, they wont. But most companies will match any contributions up to a certain %...for the company I work for, it's 6%.
They are allowed to match up to 6% of your pay, but they don't have to contribute anything there is no law saying they have to. Some employers do a 2 for 1 match and I've seen some as high as 5 to 1. In order to get anything out of the match though you have to be vested for a certain number of years, but most match up to 1:1. You can contribute up to 15% of your pay before taxes but the employer can only match up to 6%.