There are actually several noticeable differences between a guinea pig and a hamster. Although they are both rodents and are popular as pets, guinea pigs are much larger and calmer, while hamsters are smaller and more active. Guinea pigs are active during the day and are very social creatures, while hamsters are usually nocturnal and do very well as solitary pets. Many experts say that guinea pigs adapt more readily to being handled frequently and tend to make a variety of sounds. Hamsters usually need to be kept in some type of enclosure most of the time; they tend to nip when irritated, but can be very entertaining to observe.
A guinea pig and a hamster are both considered rodents, but they belong to entirely different species. The scientific name for the guinea pig is Cavia porcellus; it belongs to the family Caviidae and is sometimes referred to as a cavy. The hamster is a rodent that is classified as any of approximately 25 species that belong to the Cricetinae sub-family. Another difference that is immediately apparent is size; the guinea pig is significantly larger than the hamster.
Although they are both popular pets, another noticeable difference between a guinea pig and a hamster is temperament. Guinea pigs are usually calmer, more docile, and easy to handle, while hamsters are generally more active and excitable. The hamster is also nocturnal by nature, making its most active time at night when its owners are likely to be sleeping. It also tends to be solitary and does well without others for company. Guinea pigs are usually alert during the day and sleep at night, and they are very social animals who thrive best when they are kept in pairs.
When deciding between a guinea pig and a hamster for a pet, there are a few important considerations. Due to their size and need for company, guinea pigs need a much larger habitat to live in than a hamster. Guinea pigs also will "converse" with their owners and each other by making a variety of noises and adapt easily to frequent handling. It's even possible to train them to wear a harness and walk on a lead. Hamsters require a smaller habitat with several toys for to accommodate their high levels of activity, and they can be very amusing to watch while they play. They should always be kept in some type of enclosure since they startle easily and can be hard to find if they run away while out of the habitat; they also tend to nip if they become annoyed by a lot of handling.