Guinea pigs are a member of the rodent family who originate from the Andes Mountain area of South America. They have a lifespan of 4-6 years and usually grow to around 1kg in bodyweight. They make great pets as they are clean and quiet, and if handled regularly become very tame and sociable. However they do have some particular dietary and housing requirements, which owners should be aware of.
Guinea pigs are herbivores, and in the wild grasses form a large part of their diet. Because of this, guinea pigs, like rabbits and horses, have evolved to have teeth that grow throughout life. To prevent overgrowth it is important that pet guinea pigs are also feed a primarily fibre based diet. This is best achieved by always having a high quality grass hay available.
Another important consideration in a guinea pig diet is Vitamin C. Guinea pigs (like humans) do not produce their own Vitamin C and must obtain this in their diet. Feeding dark leafy greens like spinach, kale and dandelion leaves will ensure your guinea pig gets enough Vitamin C. Guinea pigs develop their eating habits early in life. This it is important that they are exposed to a variety of foods while they are quite young (first weeks of life).
Guinea pig pellets should have additional vitamin C added (rabbit pellets will not - which is one of the reasons it's important not to feed these). However it's important not to feed too many pellets as these will not properly wear the teeth and are likely to cause your guinea pig to become overweight. About ¼ cup pellets daily along with unlimited hay and at least ¼ cup packed fresh food daily is a good guideline for an average sized guinea pig.
Your guinea pig should always have a supply of fresh water available. This can be provided with a sipper bottle or in a water bowl. Ensure the water bowl isn't easy to knock over and make sure the water is changed daily (even with sipper tops as they may "regurgitate" into these). They like to play with their water so check the bedding around the water when changing it to ensure it's not damp.
Proper housing is very important for your guinea pig's health and happiness! Enclosures may be made of wire, steel, plastic or glass. Be careful with wood and plastics - guinea pigs are very good chewers and may destroy these. The enclosure should be in a cool dry spot with adequate ventilation. A space of 650 square centimetres per guinea pig is recommended. Bedding materials should be clean, safe, dust free and easy to replace. Wood shavings or shredded paper are useful. Guinea pigs may be housed together, they are generally social animals and will rarely fight, however new males are more likely to fight especially in the presence of a female.
For more in depth information regarding guinea pig care, including breeding guinea pigs and common guinea pig ailments this Veterinary Partner guinea pig health article is an excellent resource.