
The numbers after the letter "i" are a high level indicative of the processors' relative processing power.
i3 - less power... i7 - more power. Processing power is determined by their Intel Processor Star Ratings, which is 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. About cores: Simply, 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.