GET and POST are used to send information from client browser to web server.
GET the information is send via GET method in name/value pair and is URL encoded.
The default GET has a limit of 512 characters.
The POST method transfers the information via HTTP Headers. 
The POST method does not have any restriction in data size to be sent.
POST is used for sending data securely and ASCII and binary type's data.
The $_REQUEST contains the content of both $_GET, $_POST and $_COOKIE.