Some of us may have had pocket pets while we were growing up, but many have not had these experiences. While this is not all encompassing information, here is some basic information about some of these species.
Hamsters
Hamsters are crepuscular animals, this means they are awake at twilight and dawn. Usually we don't recommend keeping a hamster in a bedroom, unless you are a heavy sleeper, since those wheels go off when we are sleeping. There are different types of food available for hamsters, but a pelleted diet is preferable to a seed mix. Hamsters tend to select their favourite parts of the seed mix, so seed mixes predispose to obesity.
Ensuring they have lots of things to do and explore such as different items to chew in the cage, species specific toys that get rotated out to keep them excited. They'll live most of their lives in their cage, we have to make their cage fun! There are plastic balls that hamsters can ride in, but ensure to keep them away from stairs, and make sure the lids are closed tight before they take off on their ride. No hamster is born an athlete, so slowly increase the time in the ball over a few weeks, and put them back in their cage when they are tired.
Hamsters are more solitary animals, and don't usually prefer to have other hamsters in the same cage. This does mean that the humans need to spend a good amount of time socializing with them.
Gerbils
Gerbils are extremely active and definitely need to be kept busy! They are also prolific chewers, with paper towel rolls and Kleenex boxes being a great addition to the cage. Being a desert species, gerbils need dust baths, much like a Chinchilla. You can buy chinchilla dust at the pet store that they will use to keep their coat clean and dry, it also prevents their scent gland from getting clogged or infected.
Same rules of feeding apply to gerbils as hamsters- seed mixes can predispose them to obesity. Check out Oxbow for some great feeding information for both of these species. Gerbils are more social than hamsters, with their same gender, of course! They can jump, and are good at escaping- so keep those cages shut tight.
Rats
Rats make great pets, they can be quite affectionate toward their owners. Many rats even like being tickled. Rats eat lots of different types of food, though a pelleted diet is also a great idea for keeping them healthy and less likely to become obese. They'll need a lot of exercise and socialization, it is often a good idea to have more than one rat at once. Their teeth, much like a hamster or gerbil's teeth, will grow throughout life. Keeping those incisors at a normal length with items to chew such as chew blocks can help to reduce overgrowth.
For all of these species, avoid aspen wood chip bedding, since many species can develop respiratory issues with this type of bedding. Better yet, using recycled newspaper bedding is preferable to wood chip. Cleaning the cage often is paramount, especially with rats- since they do make a large amount of feces, they may need their cages cleaned daily.
These animals also need regular healthcare, like any other pet. If you think your pet is at all abnormal give your clinic a call!