HIIT = high intensity interval training
Rather than doing slow or medium paced cardio for a long time, you do HIIT for @15-20 mins. There are many ways to do HIIT. You just alternate between doing a high intensity interval and a rest intervals and then back again, based on your fitness level.
An example would be jog 1 min, run 1 min, jog 1 min, and so on. It doesn't have to running. You can do HIIT w/ any type of cardio. You can also play around w/ the length of your intervals.
It is a great fat burning workout, it's not boring, and it takes less time. There are lots of posts on the boards about interval training and more specifics.