$_POST is a PHP method that has one job – to take the info you entered into a form and make it available to the server you are sending to. It does so by storing that data in an array. The index of the array is the form fields and the corresponding value of each index is the data you entered. So for a registration form like one below

The $_POST array would look like so:
username->rambo; password->123shoot; email->rambo@hurtin.com;

You can access the values using the following syntax:
$username=$_POST['username'];

So if a url looked like:

http://www.fakename.com/index.php?target=help

and with register-globals on, php will create a variable, $target, which has the
value ‘help’ in it. It’s very useful and friendly but, if you don’t initialize
your variables then some non-so-nice person could initialize them for you by
passing them on the uri/post/cookie/etc so that your code no longer works as
expected.

You’re safe with it on if you always initialize variables, and set
error_reporting to E_ALL while testing so you catch any you might otherwise
miss.

If you leave it off, you need to use the associative arrays $_POST, $_GET etc.
The above example would be $_GET['target'].

Taken from this site

Why was register_globals disabled in PHP?
Register_globals was set to off by starting with PHP 4.2.0. When this setting is set to on, your script is automatically injected with various environment, get, post and cookie information set to variables. This poses a security risk since the environment variables could be set externally by using the URL. The following link provides a nice example of the security risk of setting register_globals to ON.
How register_globals can lead to insecure code

I sometimes get confused between the two. The following technique helps me to remember which one is which.
Get thru URL – GURL – GIRL. So the other one is Post via behind the scenes.