Crossbar fabric is a marketing term Cisco uses to describe one of the backplanes on a 6500/7600.
It basically means that each linecard is connected through to eachother via the crossbar. On a Sup720 the crossbar is integrated into the Supervisor. On older Sup2 it's a dedicated card that is placed into the middle slot(s) of the chassis. There are actually two channels per slot which go to the crossbar, with speeds up to 720Gbps per slot.
The main architectural differences between the card is that the X6748 has dual fabric channels to the crossbar and the 6516 has only one. This obviously results in less throughput to other linecards.
Hope this helps
Michael
Cisco TAC - LAN Switching