It is a poorly drawn pineapple (according to Patrick) that looks like SpongeBob's house, but is completely black and white and is missing one of the windows in the front. Other than that, the two houses are almost identical.
The interior has not been fully shown yet, but the best guess is that it resembles a drawn interior of SpongeBob's house. The things that have been seen inside is a chair and a door which both resembles their counterparts in SpongeBob's house.
Role in series
DoodbleBob escaped from SpongeBob and Patrick and drew his own house.