system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Someone explain CPU cores to me

ANSWERED (I asked this and read a few replies and then went to sleep and forgot about it. Oops) I don't get it. Is it like if I have a 2ghz processor with 2 cores it will deliver 4 GHz? Or do the 2 cores add up to a total of 2ghz? Or can it do twice as many calculations or what? I've never understood. My actual processor is an Intel i5 2.7 GHz quad core processor. Does that mean 10.8 GHz? I'm very confused.
gpt
You seem to have a bit of a misconception about what clock speeds mean. It isn't a measurement of how fast a processor is. 2.7 GHz processor means that it does 2.7 billion cycles per second. If you overclocked your CPU to double the clock speed it would be able to make 2x as many calculations per second. But comparing your 2.7 GHz CPU to another 2.7 GHz CPU does not mean they have the same speed.

But, if you got an older processor, say a 2.8 GHz Pentium 4 from about 10 years ago, and compared it to your processor. Your processor would blow the old one out of the water. It is much much faster. 

Newer machining processes allow them to make smaller and smaller circuits. A new CPU with the same clock speed as an old one can do more calculations because they're able to fit more circuits onto the chip. More circuits = more calculations even if the clock speed is the same. You can't really use a comparison of GHz as a meaningful measure of a processor's performance.

Multiple cores are just different processors. They call them cores because they're built together on the same chip, but they function like multiple unrelated processors.

Having multiple cores is a bit like having 4 people to do a job instead of one. Adding more people can speed up some jobs, but not others. Imagine you're cooking dinner. You want steak, potatoes, and vegetables. With multiple people, you can have each person cook one of the things. Because you're cooking them simultaneously, you've reduced the time of making dinner by 1/3, in comparison to cooking the steak, then the potatoes, and then the vegetables. This sort of task is one that can be easily multithreaded. Meaning it can be easily divided into multiple tasks that can be given to each processor core. Multithreaded tasks become faster with more cores.

In contrast some tasks can't be effectively multithreaded. So, instead of dinner, we're making a cake. You need to mix the ingredients, bake the cake, and then ice it. No matter how many people you add to it, you can't make the cake any faster. You cannot divide this into multiple simultaneous tasks. You cannot ice the cake until it's been baked, and you cannot bake the cake until the ingredients have been mixed. The extra people, or CPU core can only wait until the previous step has been finished. Tasks like these gain no speed with extra cores.

TLDR: CPU clock speeds aren't a reliable measure of processor speed. More cores helps multi-tasking, and only speeds up some tasks.

Edit: What's that yellow icon doing there? Also thanks!