It's an online program that allows you to draw right on the site. Oekaki's generally need Java to be installed for it to work properly and has all the basic graphic software capabilities; pens, brushes, erasers, layers, color palette, and so on.
Drawings tend to be doodles but if you become sufficient in using the program and have the patience for it, you can draw very nice art. Anyways, I just suggest you drop by the Oekaki section
and check it for your self. If you want to try your hand a drawing, then sign up and give it a whirl. It's pretty neat.