If you're looking for a prebuilt desktop system, most ads and stores will list the processor's frequency in GigaHertz (a measure of the number of cycles per second a processor is capable of). So you may see 2.5GHz Intel CPU or 2.3GHz Celeron.
It's understandable that you might assume a higher number means a better processor, but in reality even two processors that are both listed as 2.6GHz may not be equal. Newer processors do more calculations per cycle and therefore often have lower frequencies in GigaHertz than their older siblings, while still doing more calculations overall. Which leaves a conundrum: how do you tell whether you've picked a well-performing processor?
The answer lies in an entirely different set of numbers.
Intel uses the designations Core i3, Core i5 and Core i7 to describe its current range of desktop processors, with each more powerful than the last.
Within each set of processors, a larger number is better, so a Core i5 2000 is faster and more recent than a Core i5 750, for example. Technically, the more recent Core i5 processors are called "2nd Generation Core i5", but I dare you to find that listed on any newspaper advertisement.
Each generation of Intel's processors offers a variety of enhancements, and there's much more detail to it, involving the microarchitecture (how the parts of the processor talk to each other), the process technology (how thin the layers of silicon are, and thus how many transistors can fit onto each processor), the chipset (what features within the processor are enabled) and the number of cores (separate processor units) and threads (how many tasks each of those cores can handle at once). Phew.
The important thing, if you don't want to delve into all that detail, is that it's almost all captured within the shorthand of the processor model names listed above.
Numbers for higher or lower performance
Watch out, though, for letters on the end - a T or S is a low-voltage version of a processor, which will perform less speedily, and consume less power, while a K is unlocked, and can be boosted by system builders to make even faster versions than the standard processor of the same type.
As if figuring out Intel's naming wasn't tricky enough, AMD has its own designations.
Fortunately, AMD generally groups products into Vision, Vision Premium and Vision Black versions. These roughly correspond to Core i3, Core i5 and Core i7, if you wanted to compare them to Intel, but more specifically, AMD says that Vision is for basic productivity, Vision Premium is for video watching and basic productivity and Vision Black is for extreme gaming, video editing and basic productivity. So with AMD, you have a clear idea about what the processor can do before buying.