Different Brush Types And What They Are Used For

Flat vs. Round brushes
Round brushes have a tip and a belly. The tip is the thinnest part of the bristles, while the belly is the thickest. Such brushes are a great way to enhance even the smallest details, since they are very precise. When pressure is applied to the brush, the hairs move to the sides. This means that the brush can be used for many interesting effects, including the dry brush effect. Flat brushes resemble a rectangle. A wide variety of different textures can be created with this brush as well. Such brushes are great for cleaning up the edge work and adding sharpness to the shapes.
Most popular types of brushes
The Teardrop brush is very small and delicate. It is created with Sable hairs, which makes it a good choice for all paint mediums, including watercolor. This brush is appropriate for more advanced artists, because its use is very specific - for very fine details that beginners are not ready for yet.
The Fan brush is very interesting. It consists of a very thin layer of hairs, which are shaped like a fan. This brush is most appropriate for blending colors. When painting a portrait, it is used to work on the hair; when painting a landscape, it is used for creating the grass and tree branches. Everything that is thin, and does not require a lot of detail, can be made with this brush. The trick to using this brush is to make sure that each mark is slightly different than the one next to it. Otherwise the painting will look unnatural.
The Mop, does resemble an actual mop, and it is a very thick circular brush. These brushes are usually very soft, and are used to transport a lot of paint to the surface. It is mostly used to create a watercolor wash, such as a base background color. 
The Liner brush has very long hairs, and it is also very thin. The Toe of these brushes can be a point, flat, angled, etc. Such brushes have a similar use as the Fan brush - hairs, tree branches, etc. They are also used by artists for their signed works of art.