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.
C. Pun. Acoustics, Speech, and Signal Processing, 2003. Proceedings. (ICASSP
'03). 2003 IEEE International Conference on, 3, page III-565-8. IEEE Computer Society, (2003)