Swamp planet in the Sluis Van sector of the Outer Rim, near Bpfassh. It was where Yoda fled to escape the Jedi purge, masking his presence in the Force by living very close to a Dark Side cave. Luke Skywalker went there after the destruction of Echo Base on Hoth, in order to receive training from Yoda. Unfortunately, he was forced to leave to try to help his friends at Cloud City, and when he returned after freeing Han Solo from his imprisonment at Jabba's palace, Yoda was on his death bed.

There is very little that can be said about Dagobah's history - it appears to have been wiped out of star charts and historical records around the time of Yoda's arrival, perhaps by Obi-Wan Kenobi.

There is some speculation from Star Wars fans that Dagobah and Naboo are really the same planet. This seems improbable, though until Episode 3 comes out, it's impossible to say for certain.

Second and third paragraphs courtesy of www.aldera.net