What are they? --> A watch-like device, and a chest strap (for the better ones) What do they do? --> measure your heart rate while working out (many also calculate calories burned, VO2, etc) What don't they do? --> force you to get your butt off the couch What are the best ones? --> matter of taste, I have a Polar FT4 (recommended by a friend who teaches Zumba, spin, and other fitness classes) Where can I buy one? --> store that carry fitness equipment, online (I got mine from Amazon) How much? --> mine was about $70 Hope this helps! ~Nikki:flowerforyou: