What muscle groups do rowing machines work? Do they give a good cardio workout?

Hi
The rowing machine will work your rhomboids (upper back), latissimus dorsi (mid back at either side of your torso), quadriceps (front of your thighs), hamstrings (back of your thighs), your core muscles and your biceps and forearms. So a pretty good workout.
Cardiovascular-wise, it is a great. Low-impact but as it works most of the muscles in the body you can get a good calorie burn. I'd recommend it over an elliptical/cross-trainer.

As a previous Rower I have spent many an hour on an Ergo - rowing machine. I have a love hate relatsionship with them! I watch people in the Gym with their many techniques and hope that this can help you.
Muscles
It is one of the best cardio work outs.
Try 30 sec on max power and rating to get the 500m split down as far as possible followed by 30 min off for 5 mins at the end of your workout to incr fat burn and metabolism.
It works your Thighs
Calf muscles if you point your toes at the end of the slide back.
Lower back when your lock anr rock your back at the end of the stroke
Upper arms when you pull the handle into your body at the end of the stroke.
Ergo Technique
Make sure your feet are strapped in across the end of your trainer - near where the laces end.
Adjust the foot place so it is comfortable.
Start at back stops. Legs straight and arms naturally held by your side with the 'oar' in front of you
Move hands forwards first.
Then swing back to 45 degrees
Then break the legs to move forward up the slide until frontstops. (your maximum comfortable compression)
Tip the hands up so they lock in place to allow you to DRIVE the legs back.
At back stops then swing the back to -45degrees and pull the arms in.
You are then ready to tip the hands down (as if taking oar out of the water) if you want and start moving the hands forwards again....

Rowing machines are primarily CV based. You will be able to work the muscles in your legs, low back, core, upper back and arms on the rower.
It is always good to mix your training up, to keep the muscles guessing and progressing.

Rowing machines give a good cardio workout. Most of them these days allow you to train in different ways. For example, you can perform intervals on the rowing machine. The display will allow you to set the periods of time you want to work and rest for. This increases the intensity of your workout making it more interesting. Therefore, you don't need to row at the same pace for long periods in order to feel the benefits.

I used to train on the Water Rower, it gives you a good workout and it's low impact

Benefits:
Challenges coordination and cardio fitness simultaneously
Excellent for total body conditioning
Main muscles
Arms
Chest
Back
Shoulders
Core
Glutes
Hamstrings
Quads
Calves
Intensity
Moderate high
Increase resistance and/ or speed to raise intensity

Highly recommend trying 'tabata' training on the rowing machine, as well as other cardio machines. 20 secs as fast as you can (high level of resistance works effectively) followed by 10 secs active rest. Repeat 10 times. 5 minutes in total but excellent for fat burning and combines both the aerobic and anaerobic training zones.