1 | SPONGE BRUSH: The great thing about these brushes is their cheapness (I found a bag of about 10 brushes at the dollar store) and their ability to soak up excessive amounts of paint. These brushes are stellar for stencil work, especially on super absorbent surfaces like fabric. However, Picasso wouldn't be caught dead with a sponge brush. Don't plan on having a great deal of control or accuracy with these brushes (hence the reason they work well with stencils).
2 | HOME IMPROVEMENT PAINT BRUSH: This is the standard brush you buy at a home improvement store to paint or stain furniture or walls. When it comes to crafting, I recommend looking for something fairly stiff, about two or three inches wide, and as cheap as possible. Again, I recently found a package of three of these brushes at the dollar store. If you want to paint your walls, ignore everything I suggest here. However, if you want to add a worn look to a piece or create distinguished brush strokes in your project, these brushes are good for "quick and dirty" work.
3 | TOOTHBRUSH: Did I surprise you? Amazingly enough I've used an old toothbrush just as much in my art works as my crafts. I use the brush to splash watercolors and watered-down acrylics in a controlled way. The spread of a good flick is usually a 5" diameter, and it's really easy to aim so you don't end up with messy splatters everywhere.
4 | ART BRUSH (a.k.a. THE REAL DEAL): I could write 100 posts on the multitude of brushes available in this category, but I'll spare your eyes and brain. Most of my projects use acrylic, which I'll praise a bit in a future post, so I stock up on semi stiff brushes in a variety of sizes and shapes. Flat brushes are best for filling in large areas (my favorite is a 3/4" brush), and I prefer small rounded brushes for detail (I have sizes 3-9 and they all get a bit of use). If watercolor is more your cup-of-tea, I suggest softer brushes that can soak up the watery paints well.
Obviously, lots of paintbrushes exist in the world, and I wish I could try them all, but I simply haven't. If you have any tips to contribute or questions to elaborate on my suggestions, comment away!