xs (phones), sm (tablets), md (desktops), and lg (larger desktops).
Each tier of classes scales up, meaning if you plan on setting the same widths for xs and sm, you only need to specify xs.
P. Chirita, S. Costache, W. Nejdl, and S. Handschuh. WWW '07: Proceedings of the 16th international conference on World Wide Web, page 845--854. New York, NY, USA, ACM, (2007)
R. Witte, and T. Gitzinger. 3rd Asian Semantic Web Conference (ASWC 2008), volume 5367 of LNCS, page 360--374. Bangkok, Thailand, Springer, (February 2009)