Judges usually create their own method for judging, such as 5 points for content and 5 for performance. No props are allowed. Poems must be absolutely original, and poets must use a different poem in each round they make it to. Someone keeps score, and there are usually multiple rounds, in which the top scorers move on to each new round until one performer remains. Sometimes there are prizes, which range from the traditional box of Moonpies to semi-large amounts of cash.
Official rules vary from region to region.
Anyone may slam, though some slams require pre-registration. It's really very easy to do and very, very fun.