Clock buffer
Beginner
A small amplifier that re-strengthens the clock signal so it can travel further and feed more parts.
Novice
A buffer is a small amplifier cell that passes its input to its output at full strength, so it can drive long wires and many inputs. Clock buffers (and clock inverters, which also flip the signal) are versions made for clock trees, with strong drive and equal rise and fall delays, such as CLKBUF_X8 or CLKINV_X4.
Expert
Clock cells trade area and input capacitance for matched rise/fall delay, a tight delay spread under variation, and outputs that tolerate electromigration. Trees built from inverters flip the edge at every level, which keeps rise/fall mismatches from piling up into duty-cycle drift.
Explained in Clock tree synthesis (Design Flow).