Like breeze blowing through the windows on a house, ports are just hardware locations used for passing data in and out. Like the breeze blowing in and out of the window, computers send information out (and receive data in) though these windows, or ports. (Not to confuse you but computers have internal ports (for connecting disk drives, monitors, keyboards, etc) as well as external ports (for connecting modems, printers, mouse devices, and other peripheral devices). But I'm not going to geek out about internal or external ports, let's just keep it simple here. How about an example?
But why port 80? Why do web servers use port 80? Nothing special about that number - people just got together and willy-nilly decided that port 80 would be the default port on which a web server would send out its content. Don't believe me? Ok, try this out: Go to your favorite web site, let's say https://www.t1shopper.com/ but instead of typing it in like you usually would, add a ":80" after the ".com" part, like this: https://www.t1shopper.com:80/. Magic! You'll get the same web page even if you specify the port number!
So what's the advantage of having port numbers like this? Well, with publicly-agreed on port numbering, entering a port number becomes optional! Yes, it's faster - we don't have to type our fingers silly entering the port number every time. Instead of https://www.t1shopper.com:80/ we can just type https://www.t1shopper.com/ and our computers know (because of the http prefix) that we are requesting a web page and so it uses port 80 by default, without us having to type it. Aren't computers fun!
Ok, let's really geek out! Port numbers have been divided into three ranges: the Well Known Ports (0 through 1023), the Registered Ports (1024 through 49151), and the Dynamic and/or Private Ports (these are very highest ports 49152 through 65535 and usually used for receiving data, as in our example above). And who keeps track of all these default port numbers, port lists and protocols? The Internet Assigned Numbers Authority. IANA not only coordinates this but also all the worldwide domain names and IP address assignments. They're busy.
We get our list from IANA every day, we clean it and sort it and organize it so it's easier to use. The ports which are reserved or unassigned are excluded from the below list.
Here's the list of ports in use which have services assigned to them: