brushes?

So I know that there are people that swear by brushes and use theirs in almost everything they make. I just don't really know why you use them or even really what they are and how they work.

I have searched (mainly on deviantART) for GIMP brushes and have found some very cool looking "brushes" to try out and see if I can figure it out but there is one big problem... I don't know how to install them or whatever needs to be done.

If anyone out there could explain any of this to me I would be very appreciative! I'm new to GIMP still but I have worked with other programs before so I do know a little if that helps.

PS - from what I know of brushes I think the white leaves and "text" here come from a brush... I may be wrong I don't know. (picture credit to Mandy)