A bandwagon is "an elaborately decorated wagon used to transport musicians in a parade." That is, musicians play while riding on the bandwagon. This has become a metaphor for group behavior.
To "jump on the bandwagon" is to join an activity or engage in a behavior already being done by other people, as if you jumped on a literal bandwagon that is already in motion (but slowly, since parades move slowly).
Example: Sarah: "Hey, it's not too late to jump on the bandwagon. iPad 2's are out, why not buy one?" Lisa: "Hmm, maybe you're right. I'll have to think about it."
Alternatives: get on the bandwagon, climb on the bandwagon, hop on the bandwagon, and so forth. Of these, "get on the bandwagon" is the most common, but any mixture featuring the word "bandwagon" will carry the exact same meaning.