Haha, it could have been a challenge for you if you picked a challenging thing to do. Entering text is easy.
If you're doing it where users don't have separate notes, then you need only one table. There's a unique ID (which increments up 1 when someone makes a new user), a username limited to 15 characters, a password at 32 characters (all words hashed in md5 end up being 32 characters, the time they signed up, the time they last edited their notepad, and the memo text itself.
When they type in a textarea, then push submit, the query updates the memo that's linked to that user's ID. The table looks something like this:
CODE
+----------------------------------------------------------------------------------+
| id | user | pass | date | last | memo |
+----------------------------------------------------------------------------------+
| 1 | name | b36e... | 2009-10-23 18:25:30 | 2009-10-24 18:20:30 | text |
| 2 | name2 | g57n... | 2009-10-23 18:26:30 | 2009-10-24 02:25:30 | text |
+----------------------------------------------------------------------------------+
Once they sign up, the only thing that will be changed is the "last" column and the "memo" column (unless they have an option to change their username/password).
If you're following the basic example I used for the queries, the textarea could look something like this:
CODE
<textarea name="memo" cols="50" rows="10"><?php echo $memo; ?></textarea>
The fourth code up there (^) defines what the $memo variable is, by gathering the text from the "memo" column in your database, in the row that has an ID that matches the cookie on a user's computer.
I don't think that site you posted uses cookies (didn't check, maybe they do). I don't think they do, because they make you enter your password on every new page. If you didn't use cookies, then instead of running this query:
CODE
"SELECT memo FROM memo WHERE id = $id"
You'd use this one:
CODE
"SELECT memo FROM memo WHERE user = '$user' AND pass = '$pass'"
Where $pass = md5($_POST['pass'])