Intel Core i3, Core i5, and Core i7 CPUs have been around for a few years now, but some buyers still get stumped whenever they attempt to build their own systems and are forced to choose among the three.
If you want a plain and simple answer, then generally speaking, Core i7s are better than Core i5s, which are in turn better than Core i3s.
Nope, Core i7 does not have seven cores nor does Core i3 have three cores. The numbers are simply indicative of their relative processing powers.
Their relative levels of processing power are based on a collection of criteria involving their number of cores, clock speed (in GHz), size of cache, as well as Intel technologies like Turbo Boost and Hyper-Threading.
The more cores there are, the more tasks (known as threads) can be served at the same time. The lowest number of cores can be found in Core i3 CPUs, i.e., which have only two cores. Currently, all Core i3s are dual-core processors.
Currently all Core i5 processors are quad cores.