Rats are medium-sized, long-tailed rodents belonging to the superfamily Muroidea. "True rats" include the black rat (Rattus rattus) and the brown rat (Rattus norvegicus), both of which are significant to humans due to their impact on health and property. Other rodent species with similar characteristics are often referred to as rats but may not belong to the Rattus genus.

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Rats are generally distinguished from mice by their larger size. However, the terms "rat" and "mouse" are not taxonomically specific and are often used to describe various muroid rodents. For example, larger muroids are typically called rats, while smaller ones are referred to as mice, regardless of their actual species classification.

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The black rat and the brown rat are collectively known as Old World rats, originating in Asia. While larger than most Old World mice, these rats rarely weigh more than 500 grams (1 lb) in the wild, making them highly adaptable and invasive in human environments.