The Quick Access toolbar was introduced in LT 2009 as another way of putting the commands you need where you need them.
Adding commands to the QAT in LT 2009 was pretty easy -- right-click on the toolbar, choose Customize, and drag-and-drop the commands you need from the list that appears. Getting them off the toolbar, though, was a little more difficult. You had to go through the full-blown CUI editor -- not necessarily ideal, especially if you just grabbed the wrong command by mistake. (For more on the QAT in LT 2009, check out this post.)
In LT 2010, the Quick Access toolbar has grown up a lot, with several new ways to add and remove commands quickly and easily.
First up, there's a new flyout menu at the end of the toolbar containing several popular commands. Just check or un-check the tools you want to display or hide.
The second way to add a command is probably my favorite: just right-click on a ribbon icon and choose "Add to Quck Access Toolbar".
Finally, you can right-click on the QAT itself and choose "Customize Quick Access Toolbar", which brings up the same command list you saw in LT 2009.
Notice the other options in this menu as well. The first one provides an easy way to take a tool off the QAT -- no more going through the CUI editor.
The second option enables you to add a separator between icons, to help you organize your tools.
Lastly, you can use the final option to move the QAT below the ribbon, in case you like it better there.