Pilates is NOT aerobics. It's a series of resistance exercises. You do use large rubber bands sometimes, but mostly your own body weight. You can get some aerobic exercise because it's tough! For example, for the hundreds you lie on your back and raise your legs to a 90 degree angle. Then you curl your head and shoulders off the ground using your abs. Then you pump your arms a hundred times. It's tough! It's more like weight training than either stretching or aerobics, though it will improve your flexibility.
It actually was designed to help soldiers in army hospitals rehabilitate, though dancers do use it. That's why it uses springs (bed springs) and body weight for resistance... it's what was handy!
I would check out a class and not a book or video. It is possible to do it wrong and hurt yourself.