What is a swarm? It is a hanging cluster of honey bees, usually about the size of a football - often on a tree or shrub branch or perhaps on a fence or wall such as the one below. 

A typical swarm

What to do if I have a swarm of bees ?  
First of all, don't panic - a swarm is almost always perfectly safe. The bees have no thought of attacking anyone as they are preoccupied with finding a new home. 

Please visit the swarms page on the BBKA website where you will find information and photos firstly confirming that you actually have honeybees. Swarm collectors are not willing to deal with Bumble bees, wasps, hornets!
Once you are certain you have a swarm you can enter the postcode location and call a collector!

visit the BBKA swarms page