There are all kinds of technical differences between CPU within the Intel CPU series/lineup, such as core and thread count, clock speeds, Hyper-Threading and bridge but these are all technical and if you want this information, just google it and loads of technical jargon will come your way.  But for the average, non-techy person, who does not want to make a hobby or career of understanding this stuff, this it is what you should know.

Core i3: is an entry level, low-end, budget CPU.  This does not mean painfully slow or less reliable and many tasks do not require anything more than an Intel Core i3 CPU.

Core i5: is a step-up from the Intel Core i3 and is a lower to mid range CPU for anyone wanting something a little better than Intel Core i3 but still needs to keep budget in mind.

Core i7: for many years was considered the best and people demanding the best and have the budget for it, would shop for a system with a Core i7 CPU. Then Intel introduced the Core i9

Core i9: is now the highest performing CPU in the Intel I series.  It is quite new and also quite expensive.  The Intel Core i9 is more processing power than what most people need for everyday computing.

Now, please note that in each of these levels/series there are newer ones and older ones, referred to as generations and a newer Intel Core i3, for example, could outperform an older Intel Core i5.  So when considering CPU, you must consider both the level/series and the the generation or age of the CPU model.