What programs can be used for IRC (=Internet Relay Chat)?
XChat is a graphical IRC Client with a GTK+ GUI. It has a look and feel similar to AmIRC for the Amiga. Special features include the mIRC extension DCC RESUME and mIRC color, multiple server/channel windows, dialog windows, and a plugin API.
It does pretty much everything you could want an IRC client to do.
XChat-Gnome is very similar to XChat however it aims to have a more friendlier user interface and integrate better with your desktop. I find for starting out on IRC its much easier to get the hang off
sudo apt-get install xchat-gnome xchat-gnome-indicator
IRSSI is the way to go: http://www.irssi.org/
It was even in the TV show 'Numbers'; check out the video on the Irssi homepage. (P.S. That was supposed to be funny)
Quassel is another IRC client for Ubuntu and Debian based Linux distributions. It has a Qt interface. Its main feature is to be distributed, it means that you run a quassel "headless" in a server that has a permanent connection, and your desktop/mobile client connects to that server. Quassel is your best friend if you need to be always connected to IRC whenever you are!
Smuxi is a nice GNOME IRC client, which features indicator support like xchat-gnome, and can be run in a client/server arrangement where the server is always connected to IRC, much like irssi+screen.
I'm very happy with my own irc-client called "f-irc" (f-irc website).
Wrote it 7 years ago (note that I've been adding features and fixing bugs like a mad men for the last 2 weeks) and it totally suits my own needs. Navigation is somewhat different than most other clients but it is consistent and also usable when you decide not to learn all those keyboard shortcuts by heart.
For fun and kicks I added a word cloud so that you can easily what happened for the last 5 minutes (which is its (configurable) refresh interval).
I am surprised that no one suggested "Pidgin" - http://www.pidgin.im/download/ubuntu/
sudo apt install pidgin