What is a cache?

A cache is a small store of copies kept close to where they will be needed. When the same thing is requested again, the copy is served on the spot instead of being fetched from the slower original source. Your browser keeps copies of pages you visit, and websites keep copies of answers they just computed. The copies are disposable: clearing a cache loses nothing permanent. It just makes the next request slow again.

A cache is a store of copies placed close to where the work happens, and it exists because reaching the original source is slow. A request goes to the cache before it goes to the source. If the copy is there, a cache hit, it is served immediately. If not, a cache miss, the request goes the slow way to the source and a copy is saved for next time. This pays off because requests repeat: the page you reload, the file a program opens over and over. Two limits follow from the design. A cache is small, because fast storage costs more than slow storage and there is less of it. When it fills up something has to go, usually whatever was used least recently. And a copy can go stale. The source moves on, the copy does not, and the cache keeps serving yesterday's version, because it cannot tell that anything changed. Every copy therefore carries an expiry date, and once it passes, the cache fetches a fresh one. Emptying the cache does the same thing immediately, which is why tech support suggests it for a site showing outdated content. The same idea runs at every scale. A processor caches data it just used and your phone caches app content.

A cache is a pantry, and the supermarket is the original source. Nobody drives to the store every time a recipe needs flour: the flour came home once, and the pantry keeps it close to the stove. A pantry is small, so it carries what gets used, not everything the store sells, and stocking one new thing eventually pushes out another. Its contents can also outlive their usefulness. Something sits past its date until someone checks. And nothing in it is the only one of its kind. Empty every shelf and you lose no food, only the convenience, because the store still sells all of it.