Masonite. You might hear it called hardboard or fibreboard as well. What is it? It is a pressed wood pulp product that you can prime and paint on just like a canvas or board. It is great for study paintings because it is low cost, can be cut to any size, resists warping in smaller sizes and is a firm consistant surface.
When you purchase masonite, you can get smaller boards (appropriate for someone without a table saw) or you can get a board 4'x8' in size and cut it down to whatever sizes you prefer. (Hint: if you are purchasing a large board like that from a place such as Home Depot, you can have 2 cuts for free. Often I will buy it and have it cut in thirds for easy transport)
The surface is super smooth on one side and bumpy on the other. You paint on the smooth side 0 but not without preparing it properly first!
The first thing I do with my cut piece of masonite is sand the edges. Just like filing one's nails, this keeps the appearance neat and trim. Any little bumps are easily removed with a quick sand on each side. The surface side can also be very lightly sanded to give it some tooth.
The second step is applying primer. You should do at least two evenly applied coats. The sides can also be primed. I know some people who prime the bumpy back, and some who leave it or sometimes paint a corner to corner X on larger pieces to keep it from warping. Really small ones don't need the X applied.
After this is all dry, you are ready to apply paint. Some people may not like the grain marks from the primer - they can take a fine sandpaper to smooth things down again. I personally need a bit of tooth to my surface, so I leave it as is.
Applying acrylic paint to masonite is nice - it is a lot like a non absorbant watercolour paper. Paint kind of sits on the surface till it dries. This gives you opportunity to blend it into the other colours and you can be rougher to the surface than a paper. Should you water it down, you will notice that the water mark sits till it is dried, nothing soaks in.
Anyways, for something to experiment with, this is a very economical way to develop your painting skills. They frame beautifully too! If you use acrylic or oil, just remember that they don't want to be put under glass.
If you want to see some of my recent studies, you can check out my current ebay auctions, this is a great way for me to offer lower priced, high quality artwork while our economy is in a slump!