The "Purusha Sukta," a work belonging to the Rig Vedic period, is the first to talk about the classifications of humans, or the caste system. However, the "Purusha Sukta" is considered to be a later insertion to the original work in order to make such classifications sound authentic and acceptable that such a caste system was prevalent in ancient times.
According to the popular belief that exists about varna classification, brahmans are considered the supreme, in which class priests and preachers reside; Kshatriyas are kings and warriors; vaishyas are traders and agriculturists; and shudras are considered laborers who provide service to those who belong to other types of varna.
Contrary to the suggestion by earlier texts that one's birth decides one's varna, it is strongly considered that it is one's guna that determines one's varna. Gunas, or qualities, are classified into three types: tamas, rajas and sattva. Tamas is the state of darkness or inactivity, rajas is the state of action and energy and sattva concerns harmony and balance. Every individual possesses these qualities to a certain degree, but the dominant guna decides one's character and, thereby, one's varna.