Go to a class and ask the instructor, some poses will not be good for your back. Yoga is fantastic, very good exercise, I haven't found a decent class local to me for about 4 years.
I have been a Yoga teacher for more years than I care to remember...started practising it myself in 1974 when it wasn't at all fashionable!
Over the years I have lost count of the number of people that have come up to me and said how much it has helped them. Rocky is right, there are certain poses that might not be beneficial to particular back problems but generally it is a brilliant strengthener. Many favour Pilates (the 'in' thing at the moment) and that is based on Yoga exercises but concentrates on the core section.
Get a good teacher for some one to one sessions... then once you know what to do every day is best if you can manage it with maybe a small class once a week for encouragement and variety
Make sure you have a good teacher and that they know your problem. I went to a classes a few years ago and if anything it made my back worse so I packed it in.
I agree with you. Many yoga poses were created to treat your back. I love Yoga Mudra and Cobra Pose combined. The poses have a lot of health benefits despite that they are easy to do.