If you were searching for how it affects performance in a practical way I'm sure most would provide you with the data depending on the usage you're looking for.
But simple asking such generic question which you know can be searched easily is not going to get you anywhere in here nor answered in the most detailed correct way.
A cache is a block of memory on the cpu used for temporary storage of copied data already present in the main memory,essentially it allows the cpu to access certain important data quickly instead of letting the cpu look for it in the main memory first,it is a way to reduce down time and improve efficiency (in a way it is the cpu's very own random access memory)
The cache is extremely fast (much, much faster than system RAM) memory that the CPU uses to store the data that it is currently working on. Typically, the CPU cache is SRAM, which is both faster and quite a bit more expensive than the DRAM used for system memory.
The size of the cache is very important in all around computing, games, business apps, photo editing... you don't want to get stuck with something like a Celeron with 512 cache when for a few dollars more, the e5200 with 2mb of cache is much much better.