The easiest of the freestyle raps is to slow beats between 80 and 90 BPM (beats per minute). This gives the rapper enough time to think about what's going to be said next and the opportunity to use multiple syllable words more frequently.
Fast rap is a freestyle method that tests vocabulary skills as the words have to come quick. BPM is usually in the 120 to 130 range with a heavy use on one and two syllable words, though the better rappers can use three and some four-syllable words.
Environmental raps takes things in the immediate area and creates a story about them, sometimes starting with one and then linking other things to it. This can be about a person or object.
Descriptive raps spend the bulk of their content describing and object or objects using metaphors and similes. Metaphors are figures of speech that assign a description to something that is not normally possible. For example, using the phrase "Diamond eyes cast light on dark pride." Eyes can't be diamonds but the analogy makes it more potent in the mind of the listener. Similes are the same but use the conjunction like or as to make the description ("Sit like a fat cat.") Descriptive raps are full of strong imagery and are really more about the skilled use of them than the subject matter itself.
Battle raps are aimed at a particular individual and it's normal to insult or "dis" them. In a rap battle, "combatants" attack each other's clothes, hair, job, or relatives. There have been plenty of rap disputes, or beefs, in hip hop history: L.L. Cool J vs. Kool Mo Dee, Jay-Z vs. Nas, and Eazy E. vs. Dr. Dre are just a few of the most famous examples.