Dagobah is both a planet and the system in which it resides, in the Star Wars universe. It appears in the films The Empire Strikes Back and Return of the Jedi. It also makes an appearance in a deleted scene from Revenge of the Sith.
Dagobah, in the Sluis sector, is a world of murky swamps, steaming bayous, and petrified gnarltree forests. The ubiquitous white spiders that roam the swamps are actually newly sprouted seedlings of the gnarltrees that will take root and grow.
After the extermination of the Jedi and his subsequent failure to defeat Darth Sidious and save the republic, Jedi Master Yoda went into voluntary exile on Dagobah. Here, Yoda lived near a cave strong with the dark side of the Force which balanced out his good side and kept him from detection by the Emperor.
Dagobah appears to be a harsh, humid environment, mostly covered in water. It is inhabited by many predators, including reptiles such as snakes, lizards, and bats and others that are actually portrayed by a monitor lizard. None of these build or use tools; Yoda is the only civilized (and possibly only sentient) inhabitant.