The listener is the Oracle componant that (duh) listens for connections. It allows remote connection to the database, and naturually a database that can only be used locally isn't very interesting. Its part of the larger Oracle Net Services framework.
Just an FYI: In the past Oracle also used the marketing terms "Net8" and "SQL*Net" too. It's the same basic thing. Some diffrences exist but very few. See the diffrences here: http://www.orafaq.com/faqnet.htm
There are 3 diffrent interfaces to managing Oracle network configuration: Enterprise Manager, the Oracle Net Configuration Assistant GUI (the binary is netca), and the CLI tools: lsnrctl the Listener Control Utility and cmctl the Oracle Connection Manager Control Utility.
At a high level you need to do two things: configure and start the listener and then configure name resolution on the clients. Both of these are easily done using netca or EM, but we'll look at the plain files so you can do it without these tools.