The Irssi World Day
Some say the Irssi cannot be described by simple words...
"The greatest form has no shape;
The Irssi is hidden and without name.
The Irssi alone nourishes and brings everything to fulfillment."
Although there're some who tried.
"Irssi is a modular IRC client for UNIX that currently has only text mode user interface, but 80-90% of the code isn't text mode specific, so other UIs could be created pretty easily. Also, Irssi isn't really even IRC specific anymore, there's already working SILC and ICB modules available. Support for other protocols like ICQ and Jabber could be created some day, too."
However, the best approach of learning what the Irssi is can still be defined by two words: "use," "it." To this, you will find everything on http://irssi.org/.