Lets start with explaining what a port is.
I always thought that a port is those things that you plug your USB cable into, but I get the sense that I might be wrong.
Arrie, since i owe you a computer lesson, here we go:
There are two types of "ports" for input/output: hardware ports (like usb, serial, network...) and ports of software, like a webserver.
So to get data in and out over a network, you need at least one port on each of the two computers. With streaming services, you can even broadcast one-to-many through a single port on the streaming server.
There are 65535 ports, some of them are standard, or reserved, like http (web, port 80) and https (web secure, port 443). Instead of "program" you refer to a communicating software like web servers as a "service".
Remotely identifying which services (software) are running on a remote computer is one of the first steps in preparing an attack. It's like this:
1. identify services
2. identify unsecured services
3. find vulnerabilities of these services (bugs)
4. exploit these bugs and get access.
To identify a service, you just connect to the certain port with a tool like "netcat".
If there is a response, some bytes or a service header like "...apache http server...", you know that there is this specific software running on the machine, so you can take more steps to find out the version of apache server and look up exploitable vulnerabilities, or even try to find your own, undiscovered bugs to attack the service and possibly gain system wide administrative access.
A
port scan is just automating this process and scan all 65k ports to identify the "open" ones, put out a list of all open port numbers and header messages. These headers often show the software/service name and version.
Hope i'm free of debt now, finally and no one else gave you this lesson before
