My mum volunteered as an adviser there a couple of years ago, and she really enjoyed it (although I don't think she was a massive fan of the paperwork).
She helped people with all sorts of issues, there are a lot of illiteracy issues in my area, so often it would just be helping people fill in important forms. She used to give out advice on what kind of benefits were available for people in different circumstances, or how to go about getting a job. Quite a few people would come in about issues at work, or issues regarding losing their jobs, so she ended up advising people about employment tribunals etc. They cover all sorts of things though, a man came in about the domestic abuse he was getting from his girlfriend, and that was really tough because there's a lot of stigma about domestic abuse when it's that way round. Most of the time though it was just giving out advice to people in massive amounts of debt, and advising them how to go about declaring yourself bankrupt.
I think they're funded by the council. Our local CAB was closed down when the council went through massive cuts last year.