Use the selection tools to draw out the boundary of a selection.
To move the selection, click and drag on it. The main view will scroll as required to allow you to move the selection to part of the image that is not currently displayed.
You can free-form Resize the entire image or Smooth Scale the selection using the corresponding handles. Hold Shift while free-form scaling the selection to maintain aspect ratio. The mouse button invokes a context menu with common commands and Image Effects.
You can use the cursor keys while drawing out the boundary of the selection or while moving it.
If you hold Ctrl before moving the selection, then you will move a copy of it. The selection will be smeared when moving it while Shift is held.
There are two selection modes: Opaque (default) and Transparent. If you use the Transparent selection mode, all pixels of the background color will be transparent (background subtraction). This allows you to paste a selection without the background. To perform background subtraction on a dithered image, use a Color Similarity setting other than Exact.
You can apply Image Effects to a selection - see the Image Effects section for more information.