The brush engine of Photoshop is a powerful tool and can be the key to simulate real paint. But in the wrong hands it can create awful things, making your pictures not look like painted, but just like they were done in Photoshop really bad. Especially when using prefabbed brushes, where you don’t understand how they are working. So it’s always better to build your own brushes that suit your needs as best as possible.
The Photoshop brush engine allows you to create your own brushes, starting from a brush tip shape that can be just a circle or a sampled image. This shape is repeated along the stroke. To avoid this looking boring, Photoshop gives you some tools to vary these shapes along the line to get a more complex look.