Use fewer when referring to things that can be counted. Use less when referring to stuff that can't be broken down into smaller pieces. In general, people tend to misuse "less" where "fewer" should be used, but not the other way around.
- Frank Thomas has stolen less bases than Jose Reyes.
- Juan Pierre has fewer home runs than Adam Dunn.
- Juan Pierre has less power than Adam Dunn
Learn it. Love it. Use it.