/etc/services is a registry of programs and port numbers. telnet and ftp as well as other services are defined in this file and you can use this file to find the port numbers at which each of these services are available. A program that wants to reserve a port number must specify it in this file.

The format is similar to this:

service port/protocol aliases

The service is the name of the network service this entry is regarding.
The port field specifies the port number of the network application.
The protocol can be TCP or UDP and indicates the transport layer protocol that is used for the network service.

An example of the /etc/services files:

tcpmux         1/tcp
echo           7/tcp
echo           7/udp
discard        9/tcp        sink null
discard        9/udp        sink null
systat         11/tcp       users
daytime        13/tcp
daytime        13/udp
netstat        15/tcp