After Darth Vader hunted down nearly all of the Jedi, Jedi Master Yoda hid in a swamp on Dagobah where a hollow tree strong with the dark side of the Force hid him from detection by the Emperor. In Star Wars Episode V: The Empire Strikes Back, Luke Skywalker is visited on the ice planet Hoth by the ghost of Ben Kenobi, who tells him to go seek Yoda on Dagobah to be trained as a Jedi. Skywalker flies to Dagobah with R2-D2 in his X-wing after Darth Vader and his stormtroopers overran the Rebel Alliance base on Hoth. Skywalker leaves Dagobah early to attempt to rescue Han Solo, Princess Leia, and Chewbacca who he feels are in danger on the planet Bespin. Skywalker also briefly returns to Dagobah in Star Wars Episode VI: Return of the Jedi, in which he has a final conversation with Yoda, and speaks extensively to Kenobi's ghost about the conflicting stories of Luke's parentage.
Dagobah appears to be a harsh, humid environment, mostly covered in water. Given the director's penchant for portraying planets as a single environment, we can assume that the swampy marshland that surrounds Yoda's hut is indicative of the entire planet.