The question posed in the title is a legitimate one. Even though guinea pigs are familiar, they're neither pigs nor do they hail from Guinea. These rodents, originating from the Andes mountains in South America, are so similar to rabbits they were nearly reclassified. They're commonly called a cavy, a derivation of their scientific name.
But whatever their scientific status, their role as pets in the lives of millions is beyond debate. There are a baker's dozen recognized breeds and several 'unofficial ones'. But each has unique qualities that make it a far more interesting animal than you might expect.
They amuse their human companions daily with a range of behaviors that are far different from rabbits, hamsters and others. Timid, yet full of amusing habits, they wheek or whistle, popcorn or hop, and exhibit a range of distinctive behaviors unlike their mammalian cousins.
Guinea pigs are small, accounting in part for their popularity as caged pets, where they reside happily. They reach no more than about 10 inches (25 cm) long and weigh no more than about 2.5 lbs (1.2kg). They live an average four to five years, but can last as long as nine. But within that size and time they pack a lot of enjoyment for their human companions.
Some look similar to fat rats, apart from having a flat face, not a narrowed one. Others look like small, rounded rabbits, but lack the obvious ears of even short-eared lagomorphs. Many are long-haired, like the Peruvian, others are short-haired like the more familiar American. But they all share the same diet, the same propensity for developing certain diseases and the same undeniable cuteness.
They can breed often year round, producing as many as six litters, though this would be pushing the limit for a sow. Their gestation lasts about two months and produces an average of three young per litter, though several more are possible. The young are immediately active soon after birth. Though they suckle like all mammals, they can eat grass right away (one element of their natural diet). They're also fond of Timothy hay and will eat some fruits and vegetables.
Like rabbits, their teeth grow continuously so they can often be found gnawing on anything available. They do well with chew toys designed specifically with the cavy's teeth in mind.
They won't usually do much with a walking wheel, since cavies tend to be quieter and more placid than other rodents. Depending on the design, it can also harm their feet. But there are dozens of toys that can help keep them from being bored. They love to snuggle in small valleys of blanket or shavings.
Contentment is easy to detect, since they'll purr like a cat, a sound known among cavy aficionados as bubbling. Attending a professional cavy show, the sound can be obvious, when a whole chorus of these affectionate animals may 'sing' in harmony.