A dream jumper is where you quite literally, jump into other peoples dreams.
xd People have different methods of doing it, so I'm not going to post 'em up. It's basically astral projection in your sleep. To my knowledge, you have to be able to lucid dream before you can jump - though I'm not too sure. Lucid dreaming -is- acheivable. Basically, just try and remember your dreams so that you can identify that you're dreaming. To do this, keep a dream journal. ^^; Lucid dreaming is basically where you realise you're dreaming, and can control the dream. Here are some useful links:
http://www.consciousdreaming.com/lucid-dreaming/how-to-lucid-dream.htm
http://www.experiencefestival.com/a/LucidDreaming/id/21777
Hayate: yes, I am guessing that is slight empathy. ^^ Congrats!
Sophie: hmm. I shall think about this... The dreaming, I have a theory on: but to be honest, even I recognise that it's silly. Basically, in your sleep your mind mulls over what happened recently, and has fun trying to think up the 'what next'. It churns out multiple theories: like a ripple in a pond effect. Sometimes, it'll get it right and bang! De-ja-vu effect. [cough] I believe it is called precognition.