The Builder Mode is an enhancement of the vanilla Creative mode. It adds several new features that take a lot of the pain away from traditional building and make everything just a bit easier.
Many features for builder mode can be toggled or accessed through the Context Menu. The menu can be opened by holding down Left Alt. See Context Menu for more information.
To the right of the hotbar, you can find the Builder Tools. These tools are very simple and are intended to ease structural/small scale building where use of the more powerful Editor Mode is too unwieldly.
To start using a Builder Tool, scroll to the 10th slot or press 0. To switch to a different Builder Tool, hold Left Alt and then scroll.
The tools that are currently available in this section are: