There are, also, diagramming programs, that have make drawing simple geometric shapes easier to do and easier to move around and esier to edit and change their appaerance. What type of diagrams are you drawing/painting? So, there's the quick tutorial about layers. It's licensed under GPL with source code.
It is also available for free on Windows, Mac OS and Linux. This makes it an excellent case for artists or designers who may not have the budget to spend over 700 USD on Photoshop. You can, also, more easily move elements around on a layer more easily that having all the elements on a drawing or painting on one layer. GIMP is completely free and open source, meaning you can use GIMP and all of its features without spending a penny.
If you don't alike what you did on a layer, you can erase an entire layer and start drawing or painting on that layer, again, or delete the layer entirely. If you want to alter elements on a layer only that layer gets affected. You can build up a drawing or painting by putting different elements on different layers.
Think of layers as multiple sheet of clear paper or thin sheets of clear plastic that you can draw or paint on. Grab seems to be the only thing that Ive found, in the Applications folder. Why are you afraid of applications that use layers? Layers are one of the easier concepts of both drawing and painting apps to understand and use. Ive used Paint on Windows for a long time and was wondering if OS X has an equivalent app.