How does the additional row of keys above the standard keyboard work?

Using the swipeable keys

If you want to enter the character in the middle of a button, just tap on the button.

If you want to enter one of the other four characters, make sure to swipe over the button in the direction of the character you want to type.

You can see the buttons in use in the video at http://www.youtube.com/watch?v=HMOaHybJ7ZY&hd=1

Tab key

The first key in the row has some special functions:

Tap on the key to enter a tab character. If text is selected, pressing the tab key will indent the selected lines. This is equivalent to "Shift Right" in the editing menu.
The character in the top left corner is an "untab" character. It has the reverse effect of the tab character and allows you to unindent selected lines. This is equivalent to "Shift Left" in the editing menu.

The character in the top right corner is forward delete. It deletes the character that follows after the cursor and is the equivalent of the "Del" key on regular keyboards.

The bottom left character moves the cursor to the beginning of the current line.
The bottom right character moves the cursor to the end of the current line.