As in any pet care, the wild origins dictate a lot of what is important in Guinea Pig care. The guinea pig is a species of rodent that is also known as the Cacy. They are not in fact pigs, as their name would have suggested.  They come from the Andes, and studies have shown they have been breed from closely related species such as Cavia aperea, C. fulgida, or C. tschudii, and do not exist naturally in the wild.
They play an important role in both the folk culture of many Indigenous South American tribes and more importantly as a food source (how could they?). It was domesticated 5000 years ago for food and have appeared in many folk stories and are often used in metaphors.
They came to Europe in the 16th century by European traders and since then has seen common popularity as a household pet.  In the early days they were seen as an exotic pet and even Queen Elisabeth I had one. They were widely used for biological experimentation since the 17th century, giving way to the metaphor “guinea pig” for a test subject, though more recently they have been replaced by mice and rats.
They are very gentle easy to look after and are easy to handle and feed and are therefore popular with pet sitters. They are timid explorers and rarely attempt to escape. They will often make a noise like a whistle when interested, like for when the owner comes – or they hear the food container opening.
There are several breeds of domesticated guinea pigs, which differ in hair and color differences. The most normal varieties are the shorthair (or American shorthair), the Abyssinian with a ruffled coat. More specialist long hair types include the Peruvian and the Sheltie (or Silkie) and the curly long hair Texel.

So is a Guinea Pig the right pet for me?

Guinea Pigs are very companionable pets – so you need to be able to dedicate enough time for them, or else they will become lonesome and this might cause them to be stressed and go ill. This could be important if you are acquiring one for children, on the basis that the children will look after the guinea pig. Consider carefully that they typically live for four or five years, but may live for 8 years and the Guinness Book of Records has the record age for a guinea pig at 14 years.
Guinea pigs are relatively big for rodents, weighing between 700 and 1200g (1.5-2.5 pounds), and measuring between 20 and 25 cm (8–10 inches) in length. So they need quite a large cage. Though as a robust animal, this could be located in an outbuilding or a sheltered spot outside. They put up with cold better than excessive heat. Avoid wire mesh floored cages, as this can damage their feet and potentially lead to infection.
As for many pets you do not want to put two males together as they will fightfor domination when they are mature at 6 months or so. Females can however be grouped together. Obviously you do not want to add a male to the group as they breed very easily.

They are grass eaters, which they digest with the help of their long colon. Hay is an alternative to the grass in a domestic situation. Like humans they cannot produce vitamin C and will need to acquire obtrain this through their diet,   the guinea pig version of scurvy is deadly. Scurvy is often identified by sluggishness.  So feed them often fresh, raw fruits and vegetables (such as broccoli, apple, cabbage, carrot, celery, and spinach). Alternatively, but not a preferred option through dietary vitamins. They are fussy eaters when older, so with a young guinea pig feed a wide variety of foods. They feed continuously, so make sure there is hay available for them to eat.
Guinea pigs, as well as scurvy, can also suffer from respiratory tract infections, diarrhea,  abscesses due to infection (often in the neck, due to hay embedded in the throat, or from external scratches), and infections by lice, mites or fungus.

They are active throughout the day and will sleep on and off, waking for some activity. They like to groom themselves and will groom others.