Thanks for starting this post, Sugardude. I'll throw out a first response to your question, but I hope others will join in.
What HIIT means to me is varying the pace of your workout, during the workout, to include brief periods of intense, strenuous activity alternating with periods of slower pace exercise or rest. The idea is to expand strength or aerobic capacity by pushing it to the limit for short periods. I've done this with weight-lifting (including a few exercises where I either lift to the limit or push things until I feel the burn and can't eek out a single additional repetition. I've also done it when I run, where I'll move at a fairly aerobic pace, but add in some all-out sprints or hill runs that last about 20-30 seconds. If I run for an hour, I might throw in 3 or 4 of these "interval" sprints.
I really haven't researched which techniques work best. The above is based on limited reading and my own intuitive approach. I'd be interested to know what works best for others.