UHS, or Ultra-High Speed is a specification for Secure Digital cards that provides higher bandwidth than the "Class x" designations.
The following ultra-high speeds are specified:
UHS-I
Specified in SD Version 3.01, support a clock frequency of 100 MHz (a quadrupling of the original "Default Speed"), which in four-bit transfer mode could transfer 50 MB/s. UHS-I cards declared as UHS104 also support a clock frequency of 208 MHz, which could transfer 104 MB/s. Double data rate operation at 50 MHz (DDR50) is also specified in Version 3.01, and is mandatory for microSDHC and microSDXC cards labeled as UHS-I. In this mode, four bits are transferred when the clock signal rises and another four bits when it falls, transferring an entire byte on each full clock cycle.
UHS-II
Specified in SD Version 4.0, further raise the data transfer rate to a theoretical maximum of 312 MB/s.
Cards that comply with UHS show UHS-I or UHS-II on the label, and report this capability to the host device. Use of UHS requires that the host device command the card to drop from 3.3-volt to 1.8-volt operation and select the 4-bit transfer mode.
Primary Source: http://en.wikipedia.org/wiki/Secure_Digital#UHS_Speed_Class
