Basically they are premade images that you can use in photoshop with just one click, a lot of the time they are an outline,which you can use in any colour, and resize to suit your needs. Used very basically, like I have above, they look a little boring, but when you introduce colour, texture, and shading they come to life. The key is to use them with layers, colouring in the background while keeping your brush layer on the top. The images above are just a few examples of a few hundred brushes I downloaded yesterday, I went looking for wedding style brushes, and of course I got distracted by other pretty things.
Some good and free brush websites: