The UHD is the "Universal Software Radio Peripheral" hardware driver. The goal of the UHD is to provide a host driver and API for current and future Ettus Research products. There is a nice presentation about UHD at http://gnuradio.org/redmine/attachments/download/255/04-blum-uhd_presentation_gnuradio_2011.pdf. More information can be also found at UHD homepage: http://code.ettus.com/redmine/ettus/projects/uhd/wiki. The UHD can be also used with USRP1/2 as libusrp1/2 replacement.
Currently in Fedora the UHD is supported in Rawhide (F17) and it is the only way how to control the Ettus products from Rawhide's GNU Radio 3.5. Support for F16 is also on the way. In F16 both libusrp and libuhd will be supported from GNU Radio 3.4.