An IRC Client is the software you use on your computer or mobile device to access and participate in IRC networks.
There are numerous clients available for Linux, Mac and Windows machines, as well as Android, Windows and Apple mobile devices. The client you use will depend on your operating system and your personal preferences.
On MS Windows, mIRC is likely the most used client software in existence. mIRC is a very robust client with numerous features, including multiple connections, multi-network connecting, and extensive scripting ability. I use mIRC and have used it since the mid 1990s. I don't think I could switch to anything else today.
Of note to all clients: After installing a client, be sure to go through all the settings and make sure they are set properly and to your liking. Most clients let you set some text fields, like Quit/Part messages and of course your nickname, “real name”, and other info. I recommend you take any URLs out of any public fields – some channel will ban for URLs in your QUIT or PART messages, and in the Real Name field.
I will add more clients in the near future. If you have a favorite client, and would like to write a quality description and have it included here, please let me know.