Cardiac rehab Phase II (Phase I was in the hospital) - they put you on monitors throughout the exercise session (which are 3 times per week). Your BP and HR are constantly monitored during each session and your exercise levels are recorded. They include very light weights and stretching. They also have educational stuff (food, bp, meds, etc). I found it invaluable. I was a dedicated non-exerciser prior to surgery, so I had no way of figuring out HOW to exercise.
The exercises are a circuit of aerobic equipment - treadmill, aerdyne bike, elliptical, recumbant bike and nustep; whether these vary by location I don't know. You're started off slow and are lulled into thinking this was fun. They step up your exercise rapidly as your tolerance grows and have you huffing and puffing (and sweating) pretty soon. You can exercise with confidence that you won't damage your heart.
I loved it. I continue in Phase III (same place, same stuff, non-monitored, but bp and hr taken every day, RN and physiotherapists on site) and intend to forever.