Unions vary quite a lot, even within the same industry, but generally, I would say they exist to protect workers and campaign for good working conditions.
I can't agree with Loosehead's comment <<They will not be much use to you in the field of employee rights.>> I think it very much depends on the union. I do know some who have paid their membership fees for many months and then felt let down when they needed their union to advise/help them for one reason or another, but my experience was very good as a member of the Association of Teachers and Lecturers.
Some might argue that ATL is not a union but I won't go there unless you particularly want me to.
I think the best advice I could give you is to visit www.tuc.org.uk and remember that unions are not all the same.
I think it's fair to say that there has been a trend away from industry based unions, to company based ones, and older unions tend to cover workers across entire industries, whereas newer ones sometimes only take members from a particular company.
If you don't feel satisfied with the choices on offer, you could set up a new union or employees' association, and I would have thought the TUC would be only to happy to help.
I think these sorts of organisations when they work well have the potential to bring about very positive changes.
I'm not convinced by all the particular unions, but I am convinced that there is a need for unionism, or if not something similar. Some sort of employees organisation at least.
You can find out about the history of unions on the TUC website and in other places, and personally, I found that well worthwhile, even though I'm not usually that keen on history.