I guess the simplest answer is that a trade union's main purpose is to protect and improve the working rights of its members which are from a specific sector of the job force (could be anything from teachers to fast food workers to engineers, etc). They also represent their members if they become involved in a legal dispute with their employer. The most common issues are securing better pay, improving health & safety on the job and addressing various kinds of discrimination/inequality in the workplace. Members pay subscription fees to the union and are generally activists that campaign and recruit for the union as well.
Vacancies can be difficult to land as the application and interview processes are usually
pretty extensive and competitive. If you do go for it, just make sure you really put a strong effort into the application. At the interview stage (in addition to the actual panel) there will likely be a timed test such as Excel graphing, letter-writing and sometimes even role-playing scenarios. I'm only trying to prepare you - not freak you out!
I thought I came off too nervous at my interview, but it all turned out ok. I think the most important thing is to thoroughly research the union you'd be applying to and play up your interest and knowledge of their campaigns, successes, etc. Another good approach is if you ever had a job in the past where you wanted to see conditions improved for its workers - you can speak about your feelings on that as it will come off sincere, personal and relevant.
Edit: If you have no trade union experience from your home country or the UK, do not apply for anything above Clerical/Admin/Reception/Membership to start with. If you have office management experience, you can probably aim a bit higher - but don't go for Organiser or Officer roles at your first try (despite the tempting pay). You can get training for more advanced roles once in a post with the union as they generally encourage promotion and learning from within.
