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.
S. Pyysalo, F. Ginter, K. Haverinen, J. Heimonen, T. Salakoski, and V. Laippala. Proceedings of the Workshop on BioNLP 2007: Biological, Translational, and Clinical Language Processing, page 25--32. Stroudsburg, PA, USA, Association for Computational Linguistics, (2007)