Alpacas are part of a group of animals called the South American Camelids. As the name of the group indicates, they are distant relatives of the camel and share a number of characteristics. They are found in the Andes in South America and are closely related to llamas, with which they are regularly confused, at least in name.
Just to be clear, this is a picture of a llama. The ones at the top of the page are alpacas.
Alpacas are a domesticated version of the vicuña while llamas more closely linked to the guanaco. Highly prized and valued by the Inca civilisation, they were extensively bred and produced fleece that is finer than most we can achieve today.
In South America they are farmed principally by Quecha indians in the alto plana (high planes). Here in the UK, there are approaching 30,000 being kept by a wide variety of people and their numbers are still growing.
Stocking density is about 5 per acre, varying with quality of grazing and ability to rotate the animals on the land. They require some shelter but being star-gazers by nature, they will often be out and not use the shelter, even in the rain. Of course sometimes they do, just to fool you. We have found that with some of the sides Yorkshire boarded (vertical boards with small gap between) they are happy to use it. Other people will no doubt have other experience.
They are fairly straightforward to care for and people with no previous experience with livestock have been successful in keeping alpacas. That is not to say there is not a definite learning curve involved but it is manageable.