Kawaii (可愛い) is a Japanese term which means exceptionally cute or adorable. It is often used to describe "cute" hentai images, especially images of animals or younger characters (Hello Kitty, for example, though that is not a manga).
Kawaii characters are often drawn with large, saucer-like colourful eyes and small noses and mouths; they are often young, too, and will wear colourful clothes. Their hair is usually colourful and they will appear happy and shallow for a large portion of time.
The term has come into usage on online messageboards and chatrooms.