I used to do yoga every day and loved it. There are a variety of different types of yoga - some more active than others, so you can actually work up a sweat if that's your goal. There's yoga for every age and level of activity, ranging from weight loss to chair yoga for seniors or disabled.
I have a DVD called "Yoga for Weight Loss" that is a pretty good work out and it has various levels (beginner on up), so it's starts pretty slow and as you get more experienced, it gets more difficult, plus the more advanced workouts are longer.
Yoga is more like using your own body for weight training because you're strengthening your core but the poses are very deliberate. Of course, when I first started, I couldn't come close to performing the poses like the video leader(s), but it didn't take long to be able to do it.
I've actually found that I tend to lose more weight doing yoga than almost anything else. Yoga stresses balance, breathing and relaxation so it "opens" your body. When I was doing it regularly, my muscles felt a lot longer, stronger and leaner even if I wasn't actually losing pounds and as an added bonus, my clothes fit better. It also helped me relax easier.
It's always worth a try - you can research the various types to find the one you like best. There's a variety of YouTube videos to find something you like - some "instructors" are better than others, so that's something to keep in mind when looking.
You can also try both yoga and cardio - a good brisk walk in the morning and a nice relaxing yoga in the evening. Since it's a good idea to switch up your exercise routine, you could take the walk (or other cardio) one day and yoga the next, as well as learning a variety of types of yoga to make it interesting and/or different.
Yoga, as Barb mentions, comes in infinite varieties. What most of us know is pretty much beginner yoga, but if you do it for years it gets harder and harder and harder and more and more of a muscle building workout. But it's not weightlifting, you're not going to get huge biceps doing it. But if you look at the more advanced poses, they're really hard to do and require a lot of balancing on hands and arms and one leg and the like that is pretty invigorating. There are also forms of yoga that are fast and some that are very very slow. After thousands of years, and who knows how many different schools and teachers and two major religions, it's gotten quite complex. But Barb is also right that if you're fit enough, mixing cardio with any kind of strength training, which would include advanced yoga and pilates, is better than just doing one or the other. But the problem with cardio is, if you really want to do cardio you have to move fast enough long enough to get out of breath for a sufficient amount of time to strengthen your heart. I walk because I've got so much pain all the time I can't run or do elliptical or all the things I used to do anymore, but there's no way I consider walking cardio. When I could walk for a very long time, which is actually hiking, it was good exercise, but I still wouldn't compare it to running or anything else that really taxes your breathing. Walking doesn't do that unless you have an illness. One way to combine all these different things into one activity is to do martial arts, especially kung fu. Kung fu combines chi gong and tai chi, which does what yoga does for you, with lots of strength training and a lot of cardio when you do the form and especially when you spar. But it's only for the fit and for those who can take getting hurt pretty often. Peace.
Sarahjogs... I can sympathize. Both of my shoulders have been frozen - one right after the other. It took about 18 months total to get the first (right) one "thawed" including about 3 months of physical therapy and a cortisone shot. As I was working on the right on, the left one began freezing up and by the time the right one was completely usable again, the left was completely frozen. I did the same exercises I was taught in PT, but still ended up with a cortisone shot. It took close to a year to get the left one thawed...
Good luck with it.