After u synthesize ur design the next step in ASIC design flow is the FLoor planning, placement and then Clock Tree insertion . This includes all teh cells in design being placed and the the Clock connected to every cell .. so Clck tree can be said as the path of the clock from the main clock input to the cells ,... there kinds of CT like .. global and local ..
it is like a tree type of structre that will inserted at the P&R stage of the IC.which will distribute the load on the clock equalliy to reduse the delay caused by the high load present on the clock pin.
regards,
ramesh.s
Man I really was hoping some one would say its a "Tree to grow Clocks"
Ramesh you descriped it perfectly. There are different types of clock tree, like H tree or Star tree. The names are dervied from how the clock tree looks like in physical layout.