What are the different manga genres?

there is Shojo, (pretty girls with big eyes, usually teenagers)

Shojen, (boys, but other than that, same as shojo)

Kodomo, (kids 5-12 with big rounder heads and about 4-heads tall)

and

Chibi (means super-dwarfed, stand two heads tall, HUGE eyes, any age, and no nose)

hope this helps!