A packet is a small, structured unit of data transmitted across a computer network. When information, such as an email or a web page, is sent over the internet, it is broken down into multiple packets. Each packet contains a portion of the original data along with information about its destination and how it fits with other packets. These packets travel independently and may take different paths to reach their destination, where they are reassembled into the complete message. This method helps ensure efficient and reliable communication, even if some packets are delayed or lost.