Lower back pain is common affecting about 80% of people at some point in their lives. Acute back pain resolves within 6 weeks but chronic back pain, by definition, lasts longer. Sciatica implies entrapment of the sciatic nerve into the leg causing numbness and/or pain and in more severe cases weakness.
The following weblink is useful for back pain
The following weblink is useful for sciatica
Arthritis research UK has some useful advice on back exercises
For those who would like to give joga a try to address their chronic back or sciatica there is a wealth of online self help videos on youtube. I suggest these as an introduction.