TCP/IP Ports are "sub addresses" or a way to further divide an IP address and are used by services when communicating between a client and server computer.

The scheme for identifying the specific daemon or application to handle data coming in on a shared IP address is the addition of a port to the IP address. For example 120.340.560.780:80

Port numbers are divided into three ranges:

Well Known Ports (port numbers: 0-1023)
Registered Ports (port numbers: 1024-49151)
Dynamic and/or Private Ports (port numbers: 49152-65535)

Service providers (file servers, print servers, application servers, and so forth) and service requesters (clients) find out about each other through broadcasts or other means. This stage of communication lets the client and server determine each other's node addresses. Once a service is needed, a communication channel is required. Such a communication channel is made possible by identifying information that each party to the communication knows about the other. Such identifying information consists of the node addresses of each station and identifiers that describe what programs on the client and server are communicating. Successful communication employs conversations between service requesters and clients through addressing.

A common addressing method uses connection IDs, also called a connection identifier, socket, or port. These usually apply to a program on the server. For example, a Web server is typically configured to respond to communications using port 80. However, all clients communicate simultaneously with that same port. You could think of a port as being the address of an application on a computer.

The Well Known Ports are assigned by the IANA and on most systems can only be used by system (or root) processes or by programs executed by privileged users.

TCP/IP Common Ports:

20 FTP-DATA
21 FTP
23 TELNET
25 SMTP
53 DNS
69 TFTP
70 GOPHER
80 HTTP
110 POP3
137 NetBIOS name service
138 NetBIOS datagram service
139 NetBIOS
161 SNMP
443 HTTPS - HTTP over SSL

For more information on Networking Basics, the OSI Model and Server Administration:
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