http://www.rcpsych.ac.uk/mentalhealt...ments/cbt.aspx
There is some decent information about it there. Maybe show your mum that or similar to help her understand? But yep, it's about how your thoughts, feelings and behaviours are linked and it helps you work on faulty thought patterns and change behaviours which in turn is meant to make you feel better (although that is over-simplified!).
I found it really really helpful with anxiety, especially social anxiety, and with regaining control over my self-harm and negative behaviours. I didn't find it made a great deal of difference to how I actually felt but it is really helpful for a lot of people with depression etc. I found it a really positive experience overall though.