The Sudra caste is the lowest of the four castes in the caste system
in India. They are just above the Dalits
According to Hindu religious text, they were formed from the feet of the Hindu God 'Brahma' and their role was to serve the upper three castes.
The population that comprises this caste is largely a product of metisation (upper three castes mating with the untouchables and with Sudras.)
In addition it consists of those from the upper three castes who were demoted to Sudra status for violation of caste laws).
Though Sudras are not as badly discriminated against as the untouchables
they still face a great deal of discrimination from members of higher castes.
Under the caste system, the Sudras do not have the same rights and privileges as the upper caste Brahmins, Kshatriyas and Vaishyas, including prohibition of access to temples and public facilities frequented by the upper castes.
Because the caste system did not give them equal rights and dignity, Sudras converted in large numbers to egalitarian faiths such as Islam, Christianity, Buddhism, Jainism and Sikhism.
Sudra women labourers at work
Because of their historical role of servanthood, the Sudras are the ones that the Hindu fundamentalists use to commit acts of violence against the Dalits and the minorities in India.
A majority of the Indian population including the minorities comprises of Sudras.
NOTE: You can get very detailed and extensive information about Sudras and their role in the caste system in the eBook 'The Truth About Dalits'.