This is a swarm of honey bees. You can see how they hang in a cluster from the branch of a tree, with thousands of bees in a swarm. They are usually very docile and not considered dangerous if left alone.
Bumble Bees are larger, rounder and more hairy than honey bees. There are only a few hundred in a colony; often they nest in bird boxes, compost heaps and holes in the ground. There are 25 different species in the UK with slightly different markings and colours - particularly the tail colour. The Bumble Bee Conservation Trust has a useful guide to the main types.
There are about 250 species of Solitary Bee in Britain. They are generally smaller than bumble bees and quite furry, and they do not live in colonies. The Wildlife Trusts have produced some useful information on Solitary Bees on their website.
If it is yellow and black with thin wings it is a wasp (or, if quite large, possibly a hornet)