Ports are a logical endpoint used to identify specific network services or applications running on a host.
Ports are used as an interface to differentiate between types of network traffic and allow multiple network services to operate concurrently on the same device.
When combined with an IP Address, a port forms a unique endpoint address known as a Socket.
Port numbers
Ports are identified by a numeric value, ranging from 0 to 65535, and are specified in TCP and UDP.
The port numbers from 0 to 1023 are called well-known port numbers and are restricted to use for well-known application protocols (such as HTTP).
Identify service on ports
Speedguide
To find services commonly used on certain ports on can look port numbers up on speedguide.net or directly with https://www.speedguide.net/port.php?port=%3Cport%20number%3E.
Nmap
One can use nmap to identify services running on open ports on a service with a known IP Address.