While the terms "chat" and "instant messaging" are often used interchangeably, they can actually mean two different avenues of communication on the Internet. While you can chat while sending IMs with friends and colleagues, an IM is ultimately not a chat, per se.
What is Instant Messaging? In short, a one-on-one conversation in which your computer is connected to another for the purpose of exchanging text, images and more. See how instant messaging works in this illustrated guide.
Meanwhile, a chat usually refers to a chatroom, a digital forum where multiple people can connect their computers to a group of others for the purpose of sending text, images and more to everyone at once.
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