Letters for Us, Numbers for Machines
You type something like a website name made of ordinary words, and the right page appears. But computers on the internet do not actually find each other by name. Every device connected to the internet has a numeric address, called an IP address, a string of numbers that identifies it. Names are easy for people to remember; numbers are what the machines use. Something has to bridge that gap.
The Internet's Address Book
That bridge is the Domain Name System, almost always shortened to DNS. DNS works like a giant, shared address book for the internet. Its job is to take a human-friendly web address and look up the numeric IP address that goes with it. Without DNS you would have to memorize a long number for every site you wanted to visit, the way you would have to memorize phone numbers without a contacts list.
Looking Up the Address
When you enter a web address, your device first sends a quiet request to a DNS server, asking which IP address belongs to that name. The DNS system is spread across many servers around the world, and the request may be passed along a short chain until the answer is found. Once your device has the IP address, it can finally contact the correct server and ask for the page. All of this usually happens in a fraction of a second, before the page even begins to load.
Why It Usually Feels Instant
To avoid doing this lookup every single time, devices and networks remember recent answers for a while, a trick called caching. If you visit a site you were just on, your device often already knows its address and can skip the lookup entirely. This is part of why a site you visit often tends to start loading a touch faster than one you are reaching for the first time.
Source
This article was written using information from Wikipedia.