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The internet.
Alright, so to start off with, you need to know each computer gets a public address, kind of like how each house gets an address. anyone can go to it if they know it. The master list of computer (IP) addresses is kept by organizations like The Internet Corporation for Assigned Names and Numbers (Known as ICANN for short). They give these addresses out in chunks to Internet providers (AT&T, Virgin Media, etc.), that way no one uses the same one twice, which would cause a conflict.
So once your ISP gives you you personal (IP) address, you're ready to start communicating on the internet.
However, so far all you have is IP addresses. no one wants to type in 157.240.3.35 to get to facebook.
Enter the next technology: DNS.
DNS, short for Domain Name System, lets you name these addresses. For example, instead of saying "P. sherman 42 Wallaby Way, Sydney", you can instead call it "Sydney Orthodontics". In the same manner, DNS allows you to say that the address 157.240.3.35 belongs to "Facebook.com", and it automatically sends you to it. these named things that point to an IP are known as Domain Names. ICANN is also the master list keeper of these as well, as you cannot have two facebook.com sites without conflicting.
tl;dr ICANN provides many ISP's with lots of unique addresses, hands one to you, and lets you connect to anyone else connected to the same big network known as the internet. DNS allows you to not put in ugly numbers and use names like facebook.com
For a neat visualization of how corporations tie it all together, check out the Map of the Internet visualizer for Android and Apple.
P.S. you might actually want the subreddit /r/explainlikeimfive/ instead of this one.