While Nannies, Childminders and Au Pairs are all there to help look after your children, the terms of engagement are very different, and that is what distinguishes the different roles.
A Nanny is paid to come into your house and help look after the children. A nany has set hours and will generally work to a routine, but usually only looks after your children, possibly alongside her own. You effectively employ a nanny and they have certain employment rights, including the ability to take paid maternity leave.
A childminder is someone who you pay to look after your children in their own setting. They may pick children up from your home or from school, you usually have set hours and may be responsible for paying additional for any overtime incurred. They will usually be OFSTED registered and inspected, and will look after a children from various families, often of varying age groups.
An au pair is someone who looks after your children, usually in return for board and lodging and a small amount of 'pocket money' (typically less than £100 per week). Au Pair's are usually foreign nationals and often young women and men taking a 'gap year' before or after higher education and are generally looking to spend some time in this country and improve their language skills. In addition to working an agreed number of hours looking after children, they may do light housework and other chores such as cooking meals. Usually an au pair is a 'live in' position so you must have a spare room for them to live in, and you must share bathroom and kitchen facilities as required.
You will generally have a contract in place for each of these types of role, and you should look at insurance cover to make sure that they are covered for the work they do for you. All may look after children of all ages, including babies, although they are restricted by law as to how many children of different age group they may look after at once. Therefore, for practical reasons, not all child carers have the necessary space to take on your children, and they may focus on offering services to children of a specific age or attending certain settings or schools.