Definition
A web cache is a technology that saves data you've already downloaded from the internet, such as images and files from a web page, in a spot close to you. It saves you from going all the way back to a faraway server for the same data again and again.
You don't have to make the long trip to the server every time
Imagine walking to the water cooler every time you want a sip. It's a hassle, and it takes a while. If you fill a bottle once and keep it on your desk, you can drink whenever you like. A web cache works on exactly the same idea as that bottle.
When you visit a website, you download lots of pieces from a server: text, logo images, design files, and more. If you had to download all of them again from the original server in the U.S. or Europe every time you came back, pages would load slowly.
So your computer or phone saves the files it already downloaded in a temporary storage spot. On your next visit, it skips the long trip across the internet and pulls the files straight from your own device. That's why the page opens in the blink of an eye, and why you use much less data.
It lives on your phone, and along the roads of the world
A cache doesn't live in just one place. The most familiar kind is the browser cache. Browsers like Chrome and Safari save files right on your phone or computer's storage. It's a private stash just for you.
The internet also has big shared storage spaces that many people use together. These are called content delivery networks (CDNs) or proxy caches. Companies place storage servers in internet providers' data centers and at major connection points around the world.
For example, say a famous singer abroad releases a music video, and tens of thousands of people in your country watch it at the same time. If everyone connected to a server overseas, the international lines would jam up and slow down. But if a cache server in your country keeps a copy of the video, viewers can get it at high speed from a nearby server.
A little more precisely: why refreshing doesn't always change the page
To be a little more precise, a cache isn't always a good thing. Suppose the original server's content has changed, but your device keeps showing the old files it saved earlier. Then you end up looking at an outdated page.
To prevent this, web servers give each piece of data an expiration date, called a TTL (time to live). It's a rule like this: "Use this logo image for 7 days only. After 7 days, come back to the server and check whether there's a new file."
Suppose a website's design has changed, but on your computer it still looks broken or shows the old look. Often the cache is to blame. In that case, you can press a certain keyboard shortcut to skip the saved cache and download everything fresh from the server. This is called a hard refresh, and it gets you the latest files.
Common misconceptions
Clearing your web cache always makes your internet faster.
Clearing the cache frees up storage space. But the next time you browse, every image and file has to be downloaded from the server all over again, so the first load actually gets slower for a while.
Where you meet it
You return to an online store you've visited before, and the logo and background image appear instantly.
A website's design has been updated, but old images still look broken on your screen, so you're told to clear your browser cache.
In one sentence
A web cache copies the internet data you use often to a nearby spot ahead of time. That makes pages load faster and wastes less data.