Core i3, Core i5, Core i7 — the difference in a nutshell
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.
Image credit: Intel.
Their relative levels of processing power are also signified by their Intel Processor Star Ratings, which are based on a collection of criteria involving their number of cores, clockspeed (in GHz), size of cache, as well as some new Intel technologies like Turbo Boost and Hyper-Threading.
Core i3s are rated with three stars, i5s have four stars, and i7s have five. If you're wondering why the ratings start with three, well they actually don't. The entry-level Intel CPUs — Celeron and Pentium — get one and two stars respectively.
Note: Core processors can be grouped in terms of their target devices, i.e., those for laptops and those for desktops. Each has its own specific characteristics/specs. To avoid confusion, we'll focus on the desktop variants. Note also that we'll be focusing on the 2nd Generation (Sandy Bridge) Core CPUs.