The Hindu caste system includes four official levels: the Brahmins, Kshatriyas, Vaishyas, and Shudras. The Brahmin class includes the Hindu priests. The Kshartriya class includes rulers and warriors. The Vaishya class includes people most would refer to as "middle-class," including farmers, craftsmen, merchants, and other landowners. The lowest official class is the Shudra; Shudras include servants and other common laborers.
However, the Hindu caste system has an unofficial fifth level; people in this class have even less esteem than the Shudras. This fifth class was historically referred to as the "untouchable class" and have more recently become known as Dalit. Since these people are not members of the official caste system, they reside outside of it; historically, the untouchables were thought to defile members of the four official castes (hence they called them untouchable).