A small linked picture which represents an action. The icons on your desktop represent what will happen if you click on them - so a text document might be represented as a piece of paper with words on it, or an image might be represented by an icon showing a photo frame.
Internet Relay Chat. A real time chat system using a program that allows you to talk to other people on topic related channels by typing your comments in a chat window. IRC used to be very popular but is now used mainly by people with specialist interests, such as software development.