An instant messenger, also known as a "chat client," is a program that allows you to send text messages instantly to other people on the Internet. When you are having a text message conversation with someone, it is called "chatting." This is where the term "chat room" comes from. (Chat rooms usually involve multiple people, and can be either public or private.) The acronym "IM" stands for "instant message." For example, "I will IM you later."
Most instant messengers share some common features. These include the ability to create friend or buddy lists, so you can see which of your friends are online. Other features include web-cam capabilities, allowing you to see the person you are chatting with. Some instant messengers can even allow you to have voice conversations too.