An IRC client is a piece of software that enables a user to connect to an IRC server and to chat without knowing and using all protocols basic IRC commands manually (as it is possible over telnet). There are many free-to-use chat clients aswell as shareware available for a great amount of different operating systems and platforms. The number of IRC clients is still growing by today.
Categories of IRC clients
This is the most common occurance of IRC clients and used by most advanced IRC users. The IRC client is installed and executed as a standalone application that runs independently, which allows them to offer a broad range of features, comfort and speed with which the other client categories usually can't catch up with. They aren't especially known for being straightforward to use, but offer the most deep IRC experience and often also scripting languages that allow for easy crafting of additional features and extensions.
Instant Messaging Suites
The graphical standalone clients are probably the most widely used because they offer the combined control of mouse and keyboard with a, in most cases, modern and mouse-driven user interface. Popular examples for graphical standalone clients are mIRC and HydraIRC for Windows, XChat and KVIrc for cross-platform clients and Konversation and Kopete for Linux.
The text-based standalone clients are mostly not as intuitive as their graphical counterparts and often rely solely on keyboard control, but offer a more simplistic and often faster to use view on IRC. They are often used by very advanced computer users and are also popular for their easy use over an SSH terminal. Popular examples are irssi or ircII.
The mobile clients are clients specifically crafted for devices like mobile phones or other sort of mobile devices, handhelds etc. Due to the small screen size and limited resources aswell as the often differing processor architecture, they mostly need a specific interface and specific code crafted for the particular device. Most mobile clients also happen to be graphical ones with a graphical user interface. One example for a mobile client would be jmIrc.
Some popular non-IRC software, e.g. the web browser Opera or Unreal Tournament 2004, has built-in IRC clients. They offer IRC chat out of another software without the need of an additional program.