Snap with a Mouse
Select the title bar of the window you want to snap, and drag it to the edge of your screen. An outline indicates where the window will snap to once you drop it. Drag it to the left or right side of your screen depending on where you want to snap it to.
Snap with Snap Assist
Snap Assist appears after you’ve snapped a window but have a lot of available space on your screen. Any other open windows are displayed in that space as thumbnails.
To use Snap Assist, click the thumbnail of the window you want to open in the empty space on your screen. If you have two windows displayed side-by-side, you can resize both windows simultaneously by selecting and dragging the dividing line. Resize the window on one side to the size you want it to be, and then release the mouse button. The other window will resize itself to fit alongside the first one, so you won't have any empty screen space.