Hopefully this will help
i3 - 2 cores (not designed for gaming, avoid these)
i5 - 4 cores (this is mid range and can be used for gaming, it lacks hyper threading normally)
i7 - 4 cores (this is high range and usually the choice for gamers, it also have hyper threading)
i9 - Can have anything from 6 to 18 cores, these are very high end and pricey
The next numbers after the i3,i5,i7 will be a 3 or 4 digit number such as i7-7700
Knowing exactly what this means isn't important, Just know the first number "7" is the generation, the higher the number the newer the CPU, at time of this post we are on 9th gen. The rest of the number is the SKU in that generation, normally again higher is better and faster.
Now after that number you sometimes get a letter, this is the product line suffix, here are what they mean.
K - Unlocked which means it can be overclocked
G - Includes discrete graphics, this is mobile CPU only
U - Ultra low power, this is mobile CPU only
H - Includes high performance graphics
Q - Indicates a quad core card where normally it wouldn't be
T - Power optimised
M - Mobile version of a desktop CPU
Now it is really important to remember that even if a CPU fits a LGA1150 socket it may not support your motherboards chipset. So there are two things to check when buying a CPU to go in your current motherboard. You need to check the CPU specs to see if your motherboard chipset is on the supported list, and also you should check if a BIOS update is required as well. For example 7th Gen processors fit in the exact same socket as the 8th and 9th gen but they do not work on all the same chipsets. If the chipset is listed your CPU will work.
When choosing a CPU use this website
https://www.cpubenchmark.net/cpu_list.php, to compare the speed of models (order by passmark). Sometimes you will find that going to the next generation of your same processor will be faster than getting an i7 in the older gen.
If you need more help let me know
Windows 10, Intel i7 4.2 GHz, 16GB DDR4, NVIDIA GTX 780 4GB Superclocked (Passmark: 8056)