You can use POST, GET and UDP (and more).
I use a simple GET command.
My textarea is like this:
<form name="myform" method="get" action="?">
<textarea name="D" rows="3" cols="45" autofocus=""></textarea><br>
<input value="submit" type="submit">
If the 'submit' button is clicked, the webpage with the 'D' command sends the contents of the textarea.
If you pass text, it is URL encoded.
I made an URLdecode : http://arduino.cc/forum/index.php/topic,135969.msg1022648.html
So the command would be like this: GET /index.htm?D=47
After the '?' is the command (I use a single character 'D'), and after the '=' is the value.
You don't need the form name "myform", I use it to prefill the textarea with text.
You don't need "autofocus="", I use that to set the cursor in the text field.