What is a catgirl manga?

Cat girls-

Neko no on'nanoko(that's how you pronounce it.)

A catgirl is a girl (usually a heroine) that shows characteristics of cat and human, for example, cat ears, and a cat tail, can commonly be used with the rest a normal human body.

A catgirl manga is a (Shoujo) manga where usually the heroine is part 'neko' (that's cat in Japanese) and part person.

Note: In Neko no on'nanoko manga, sometimes the character can say 'nya' which in Japanese is the equivalent to 'meow'