Herobrine is a community-made glitch entity that is not a part of Minecraft. Many people claim he is Notch's dead brother, but Notch himself stated that he isn't.
Herobrine tends to build things in the worlds he infiltrates. For example, things like these have been seen:
-Pyramids made of varying blocks
-Tunnels lined with redstone torches
-Signs with varying text
More has been reported, but these are most often reported and most likely to not be hoaxes, as many people claim they've seen Herobrine but actually haven't.
Herobrine also tends to destroy things such as:
-Mostly your house, chests, etc.
-Most of the time, the glass windows in your house.