Bubble lettering and tagging are the two most popular graffiti styles among the beginners. In bubble lettering round letters resembling merging bubbles are used. Tagging is putting tags as signatures. On the other hand the complex graffiti styles are wild style implying tangled and illegible letters which can only be read by the writer.
Developing an individual style is the most important thing for gaining respect of other writers in the world of graffiti. Without your own style, it is really hard to attain recognition.