A group of employees in a particular sector, whose aim is to negotiate with employees over pay, job security, working hours, etc, using the collective power of the members.
They are funded by subscriptions from members, and often offer legal advice and strike pay during disputes when members refuse to work.
In general, the union is there to represent the interests of its members, and may even engage in political activity where legislation effects their members.
In the UK, most unions belong to the TUC (Trades Union Congress), which represents many unions, and hence has more power over issues effecting workers nationally.