CPUs are classified as per features present in them, which includes
System Bus – System bus speeds CPU supports. Current Intel processors support 1600, 1333 and 1066 and AMD CPU support 1800 and 1000 MHz system bus.
Speed or Core Frequency – It is measured in gigahertz, like 3.2 GHz and older CPUs was in MHz.
Socket Type – CPUs are mounted in different CPU sockets on the motherboard and the chipset present. Newer Intel CPU supports LGA771, LGA775 and 478 sockets while AMD’s support AM3, 754 and 940 sockets.
Number of Cores – It refers to multiple processors installed in the same CPU package and as per number of cores or processors in a CPU they are called dual core(two cores), triple-core (three cores), quad-core (four cores) or hexa (six cores).
Multiprocessing Support – It refers to two or more CPU installed on same motherboard in different sockets like Intel’s Xeon
Amounts and Levels of cache – Memory in CPU die is called Level 1 cache (L1) but memory in the processor package, but not on CPU die, is called Level 2 cache (L2) and some CPU have third cache far from CPU core, but in the processor package, which is called Level 3 cache (L3).
RAM type support The type of memory (DDR, DDR2, or DDR3) installed on the motherboard that CPU can support. Both must be compatible.
Technology support CPUs may support newer technologies like Intel’s Hyper-Threading or AMD’s Hyper Transport which allow parallel handling of computation in CPU.
Operating Voltage and Power Technologies – Operating voltage of CPU is the voltage needed for its operation and low voltage CPU are gaining. Technologies for low power consumption are used by putting CPU in sleep state when inactive like Power Now! from AMD and Speed Step Technology from Intel.