Tootler, there are different tariffs for different types of entertainment. A PRS licence for playing recorded music won't cover a live session, and vice versa.

Mr Happy, PRS is not a government organisation, however it is established to manage composers' copyright interests, which are protected by law. Whilst one can be very critical of the manner in which it operates, the alternative would be that in order to perform a copyright piece you would have to get permission from the copyright owner, and negotiate a fee in each case. PRS is probably the most practical solution. A similar model is used in most countries.

More information is on the PRS website