Difference:
An HTTP POST request is used to add new information to a server so it can be stored, and an HTTP GET request is used to retrieve stored information from a server.
Usage Examples:
Let's say you run a web server dedicated to storing football game scores.
A computer at a game that just finished may use an HTTP POST request to send the final score to your web server so you can store it.
Someone at home may be viewing a website for sports scores. That website's server may send an HTTP GET request to your server asking for the final score of a specific game, and your server would provide that information.