Students really struggle with this, even in CS 304. It's actually somewhat complicated and multi-faceted. Here are a few short things:
There's a difference in format: form data in the URL (GET) vs in the BODY of the request (POST)
There's a difference in usage: GET is used for retrieving (GET-ing) data vs POST is used for submitting (POST-ing) data.
Consequently, browsers and servers treat them differently with respect to resubmission.
If you want to really dig into it, you can read my cs304 material on GET vs POST
Great!