Window snapping is a very powerful feature in BetterTouchTool. It allows you to easily arrange windows on your Mac by e.g. using a normal mouse, multitouch gestures or keyboard shortcuts.
Basic Window Snapping Setup
When opening BetterTouchTool for the first time it'll ask you whether you want to use widow snapping. If you answer this with yes BetterTouchTool will enable the mouse based window snapping for you.
If you chose no, but now want to enable window snapping, go to Preferences -> Basic Settings and check the Enable window snapping checkbox.