Web pages send Form fields either using GET method in the URL parameters, or POST method in the request body.

Parameter of GET requests are saved in logs files, and have limited length.
Parameter of POST requests do not appear in the logs, and are not limited in length. It is thus possible to upload large files.
These parameters are encoded with 'Percent encoding'.
Example:

POST /login.php HTTP/1.1
Host: 10.10.1.11
Content-Type: application/x-wwww-form-urlencoded
Content-Length: 24

login=James&password=007

Post a form with curl:

$ curl -X POST -F 'username=admin' -F 'password=megapassword'  http://localhost:8001/