I was on JET from 1996-98. The application process is quite long... you send in your application in late Nov/early Dec, if you get an interview that takes place in Feb, then if you are selected there are orientation meetings etc and you fly out in late July/early Aug. They take care of your airfare there and back, your visa, training in Tokyo and more in your prefecture, and will help you find a place to live... some schools will actually pay for all your housing, others will partially subsidize it, and others will just find the place and you pay for it yourself. (I and most others tend to have the latter arrangement). The pay is 3.6 million yen per year, which at current exchange rates is roughly $40,000, and that's tax free in both Japan and the US. You sign up for a minimum of 1 year and renew a year at a time, up to 5 years now (used to be 3 max). You team teach in Japanese public schools with Japanese teachers, and you don't need to know any Japanese to apply (I didn't know any when I went). JET has a great support network and voluntary events/activities set up by the AJET chapters.
If you have any questions beyond the info blurb above, let me know, or go to the official JET forums here:
JET Programme :: Index