Rats are rodents and belong to the same family as mice. Many people think about calling the exterminator when they see either. It is true that wild rats can carry disease and it may be entirely possible that they contributed on some level to the spread of the Bubonic Plague. One has to keep in mind though that all wild animals can carry and spread disease. Humans have nothing to fear by the domesticated rats that are sold in pet stores.
Rats are generally very people-oriented.
They are extremely friendly and tolerate many hands-on encounters. This is why with proper teaching they make great pets for children. They rarely bite and will not get squished as easily as a pet mouse or hamster. A curious rat can be taught to come when you call its name, provided you provide treat reward. Some rats prefer to remain perched on the owner's shoulders.