There's often general confusion between different types of bees, and I thought it might be helpful to clarify who's who in a brief introduction. In the U.K. we have honeybees, bumblebees and solitary bees, and each are quite distinct. They all collect pollen (for protein) and nectar (carbohydrates) from flowers, but live in different ways and have different problems.
There's one species of honeybee in the UK - the European or western honeybee Apis Mellifera (Apis = bee, mellifera = honey producing).They can look darker or yellower, though, as there are different strains around. They are social bees, living in highly organized colonies of up to 60,000 bees, where they store large amounts of honey to eat throughout the winter. They travel up to 12km to find the nectar and pollen they need, and they forage in groups. Each colony usually has a single queen, who typically survives for several years. These are the bees we keep, and are most commonly mistaken for wasps.
There are currently 25 different species of bumblebee in the UK, although only 7 are common (including a recent import from France). Different species have different length tongues, adapted to collect nectar from different sorts of flowers; longer tongued bumblebees can access flowers honeybees and solitary bees can't. People notice the queen bumblebees in particular, as they look for nest sites in the spring after hibernating or when the new queens fatten themselves up in the autumn (as in the photo). They're big bees! Most bumblebees nest underground, often in deserted mouseholes or in places like compost heaps, and colonies are very small - only a few hundred. They're not as well organized as honeybees and produce no food stores to over-winter on; the queens only last a season and the colonies break up in the autumn. They're not as mobile either, and forage on their own over much shorter distances. They're visually easy to distinguish from honeybees; they're much furrier.
Astonishingly there are over 240 species in the UK. Hardly noticed, they go busily about the garden, typically nesting in the ground (mining bees), in wall crevices (cavity nesters) or in decaying wood (carpenter bees), according to the species. They can do well in the solitary bee boxes you can buy or make. They have relatively simple and short lives, which are concentrated on producing and safeguarding their eggs. These are sometimes protected with physical barriers constructed by the bees, typically made of mud or leaves - cut in neat half circles from plants like roses. We know remarkably little about solitary bees, which are thought to be the major pollinators of flowers in the UK. Oh, and they're stingless.
There are lots of flies which mimic all three types of bees for their own sometimes nefarious purposes. Hoverflies are often mistaken for them, but once you've got your eye in you'll be able to see they fly very differently; hoverflies...er... hover.
Honeybees, bumblebees and solitary bees have their own diseases and parasites, which are much more problemmatic when in combination with the issues below. The best known is probably the varroa mite, which has done terrible damage to European honeybee populations - to the extent that there are pretty much no wild colonies left in Britain. It is now treatable in the apiary, but colonies need careful monitoring. There are a cocktail of other nasties which effect all bees more or less, depending on your point of view:
1. Habitat loss: there are fewer flowers around and fewer of the right sorts of flowers, particularly wildflowers. Bees also need a variety of pollen and nectar sources pretty much throughout the year, from early bulbs to winter flowering shrubs and trees. You can do your bit to help by planting some bee friendly plants, particularly those flowering outside the summer months. Different flowers are good for different bees; I'd recommend Plants for Bees as an invaluable guide.
2. Climate change: warmer winters mean bumblebee queens break hibernation and honey bees become active before spring - i.e. before there are flowers out. The queens use up their fat reserves too quickly and the honeybees their honey, so they starve to death. This can be the single biggest cause of honeybee deaths in a bad year.
3. Pesticides: This is the debate which currently causes the most heat and noise. We're now using systemic pesticides, neonicotinoids, whose introduction in different countries seems to have coincided suspiciously with dramatic declines in bee numbers. As they're systemic they effect any insect that feeds off treated plants, which unfortunately includes the insects feeding off the pollen and nectar these plants produce. There's mounting evidence suggesting the damage to bees from them is significant.*
*Since this post was first written, in 2012, this evidence has continued to accumulate. Neonicotinoids seem to have affected a wide range of animals very badly and their impact will take many years to properly evaluate.