| Merriam-Webster defines "cache" as:
1 a : a hiding place especially for concealing and preserving provisions or implements b : a secure place of storage
2 : something hidden or stored in a cache
3 : a computer memory with very short access time used for storage of frequently or recently used instructions or data -- called also cache memory
In this context, we should probably focus on the third definition. Even within that scope, however, there are several different usages and meanings of the term "cache."
A web cache stores data that users browse over the web. Web caches can exist on individual PC's, or they can be seperate devices that reside on the network. When I browse a web page, Firefox stores a copy of that web page on my hard drive. When I go to look at that web page again, Firefox retrieves the web page from my hard drive instead of going all the way across the Internet to get that web page. The makes my Internet connection appear faster.
A disk cache is a section of RAM (Random Access Memory) which is reserved to temporarily store data from from a disk drive. There are two kinds of disk caches, read caches and write caches. Read caches improve the performance of disk reads, because the second and following times that you read the same data, it will be read from RAM instead of from the disk. A write cache improves application performance, because the computer tells the application "Don't worry, your data is written, get on with your work" and then later goes through the steps of actually writing the data to the disk.
Modern CPU's contain a portion of very fast RAM built into their silicon. This very fast RAM is used as a cache for the slower RAM connected via the motherboard.
Looking at these three types of caches, we see that a faster storage medium is used to cache data for a slower storage medium. RAM on the CPU is faster than RAM on the motherboard, which is faster than a hard disk drive, which is faster than an Internet connection. | 
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