Instructions Per Clock is a technical term to describe one aspect of a processor performance: the average number of individual assembler instructions execute for each clock cycle.
A processor performance can be computed multiplying its clock speed by the IPC: the same performance can be derived from a high IPC and a relatively low clock speed (like the AMD Athlon, HP PA-RISC, SUN SPARC ULTRASPARC), or from a low IPC and higher clock speed (like the Intel Pentium 4 and DEC Alpha).
High-IPC, low-clock speed processor designs are traditionally called braniac designs, while low-IPC, high-clock speed are often called speed-demons. Both of them are valid processor design techniques, and the choice between the two is often dictated by engineering constraints, marketing pressures, etc.
In particular, the speed-demon design of the Intel Pentium 4 is often cited by AMD fans as "cheating", since the higher clock speed rating makes the processor appear faster to the general, non-expert audience. It is suspected that this is the real reason of AMD PR-rating scheme, where the name of each processor is not equal to the real clock speed anymore.
