From Linuxintro

In TCP and UDP networking, a port is a two-byte number intended to define to which service network packages will be delivered. As example, port 80 is for web browsing content and port 22 is for ssh.

To find out what ports are open (listening) on your system, use nmap.

To find out what connections are established on your system, use netstat -putan or lsof -i.

To create a socket on your system, use netcat.

To test a port use telnet. For example testing the IMAP port 143 is described here.