IRC stands for Internet Relay Chat. It was originally created by Jarkko Oikarinen as a replacement to the UNIX "talk" program back in 1988. IRC is a client/server system. The user runs the client which in turn runs with the server who connects the user, the server, and the rest of the IRC network. IRC is a multi-user chat protocol that allows many users under there own "nicks" in IRC "channels" where they can converse and more. To sum it up for all, IRC is a place where many users can talk with each other. IRC is quickly becoming one of the more popular Internet Resources.
A client is the program that runs on your computer, managing the data IRC sends and receives. Under Windows, the IRC client is typically a WINSOCK-compliant application. This means you can use it on any WINSOCK 1.1 compliant TCP/IP stack (like Trumpet WINSOCK, Microsoft Wolverine, or the TCP/IP stack in Windows 95). Some of the IRC clients for Windows are mIRC, WS_IRC, IRCII for Windows, and IRC4WIN. Out of those, the number one is mIRC. mIRC supports customizable pop up menus, alias, and remote tools (simple yet powerful scripting), a toolbar, great options, and much much more. Plus, it comes in both 16 and 32-bit versions. But, if you plan on using IRCII scripts like LiCe or Phoenix, you will need a fully IRCII compliant client like IRCII for Windows, since mIRC does not fully implement IRCII nor IRCII scripting at all.
Yes, there is. EFNet and UnderNet. You see, a few years ago many operators of IRC thought that IRC needed some improvements. Many servers did not wish for a change, so these operators went out and created a new, separate, arguably better IRC from the original called UnderNet. The old servers are known as EFNet. UnderNet's new designs make it an easier, faster, and more friendly place than EFNet. But, because EFNet was the original IRC network, it is bigger and has more users. The ultimate choice comes up to you. To connect to UnderNet, just connect to an UnderNet server instead of an EFNet one. Some UnderNet servers include us.undernet.com and boston.ma.us.undernet.org.
Sometime when you are on IRC there is a long period of inactivity. Then suddenly, many users are quickly disconnected from the server. This is a net-split. A net-split is when the connection between two servers is broke, therefore rendering all the servers hooked up to those disconnected from each other. Once the two servers are reconnected everything will go back to normal (hopefully)!