GET and POST are two fundamental HTTP methods used to transfer data between a client and a server. GET is primarily used for retrieving data from a specified resource, appending data to the URL as query parameters. It's idempotent, meaning it can be executed multiple times with the same result, and is less secure due to exposed data in the URL. On the other hand, POST is employed for submitting data to be processed by the server, sending data within the request body. It's not idempotent and offers better security as data isn't visible in the URL. While GET is suitable for fetching data and can be cached, POST is preferred for submitting forms, uploading files, or performing actions that modify server-side state.