It seems weird that a dishwasher would need to be cleaned even though it is constantly being soaped and rinsed along with the dishes; however, over time it does start smelling funky and not looking it's best.
It's as easy as this:
Place a cup of white vinegar, full strength on the top rack and sprinkle a cup of baking soda on the bottom. Run it through the hot water cycle and this combo of products will do the trick to remove grease and smells.
This is a good thing to do once a month or, if you don't use it as often, every other month is fine. Happy (clean) dishwashing!