A "Brony" is a fan of the Hub television show "My Little Pony: Friendship is Magic". The majority of them are male, and are around the age of 21. Most of the bronies on the Sausage MLP Forums are preteens and young teenagers, so it doesn't reflect the demographic very well.
Following the creation of the fourth generation of My Little Pony, an article was created on CartoonBomb claiming that this was "the end of a creator-driven era in TV animation." This was noticed by many users of 4chan, who decided to watch the show themselves to see what it was like. In an ironic twist, many of said users genuinely liked the show and began posting countless pictures of ponies all over the board. In the process the term "brony" was coined as the name for all people that were fans of My Little Pony: Friendship is Magic. As a result several other users expressed much hatred toward bronies. A flame war was started, reaching the point where pictures of ponies had to be temporarily banned from the image board.
This influenced bronies to start creating their own websites, and the show's popularity began to spread among the rest of the internet. Some of the more popular websites include Equestria Daily and PonyChan. If not for the initial opposition against the show, the fanbase may not have grown into what it is today.
Bronies today are considered one of the biggest fanbases on the internet, and the population continues to grow every day.