SketchBook Express is a very good free painting and drawing application. It is an app that is much more complete than, lets say Paint, but less complete than Photoshop. It can be used to create beautiful drawings and graphics if you have the right set of skills. The application certainly provides all the tools that you need. There is support for layers, a vast number of brushes that you can use, undo and redo, and many more features.
SketchBook Express has a nice interface. It is fast and responsive and designed so that the average user will understand it quickly. It consists of a page where you can draw, and several floating panes, where you can change colors, manage layers and change tool properties. There is a half-circle at the left-bottom corner that lets you change settings as well. From there you can choose between the different brushes (pencil, airbrush, eraser, ball-point pen, marker, etc).
From the Preferences window, you can further customize the application. You can set the number of undos and redos that you want the app to remember (default value: 30), set the default format, and change some brush settings.
In short, this is a really great app to draw on your Mac. It definitely requires some skill to draw anything complex, but the app provides all the tools that are needed for that in a nicely designed package.