Shudra (Śūdra) is the fourth Varna or caste in the traditional and historic Hindu society. Their assigned and expected role in post-Vedic India was that of laborers. The four Varnas are Brahmin, Kshatriya, Vaisya and Shudra. Sikhism does not accept the caste system and a Sikh is prohibited to take the persons caste into consideration in their dealing with society.
Whilst the origins of the other varnas can be traced to Indo-Aryan or even Proto-Indo-European words, the root of the word is not clear at all. A threefold division of societies can be found in ancient Iran that matches the Brahmin, Kshatriya and Vaisya varnas. However, the linguistically related Nuristani people in neighboring Afghanistan have a class equivalent to the Shudras amongst them. 
- See Wikipedia article on Shudra for more information