It is similar in shape to a Port-A-Potty. It is painted yellow, has a wooden door with a string handle, a netted window on the door and right center of the bathroom, and a clam sign with red lettering similar to that of the Krusty Krab's.
SpongeBob takes Bubble Buddy to Goo Lagoon, and then places him inside the portable bathroom since he supposedly has to go, much to the frustration of the people waiting in line to use it. He leaves him in there for 2 hours.
It makes appearances in several other episodes taking place at Goo Lagoon.
Battle for Bikini Bottom
It can be seen in the Kelp Forest when SpongeBob and Patrick go inside of it the door close for a cople of minutes then opens.
The SpongeBob SquarePants Movie video game
The player can use it to switch from one character to another, the purpose being that the player may need to switch into another character in order to defeat an enemy with a certain attack or go past obstacles that can only be crossed by using a certain character's powers.