The Gimp has 6 main tools for selecting regions on your image.  For most applications the most useful tools are the Bezier path and the Lasso.

 
 When you have selected an area in 'The Gimp', this is the only region that your tools will have any effect on.

So if you select a small rectangle, then paint wildly over your image with the paintbrush, your paint will only appear inside the selected rectangle.

The bucket fill tool fills a selected region.
Sometimes it seems that your tools are having no effect.  This may be because you have a very small area selected somewhere that you cannot see.

An easy way to overcome this is to get the rectangle selection tool and select your entire image.

Rectangle and Ellipse

The rectangle and Ellipse work as expected to select areas by clicking and dragging.

To select a perfectly square or circular region, begin the selection by clicking and dragging in the image window. While the mouse button is down, press the Shift key. The Shift key must stay pressed until the selection is finished and the mouse button has been released.

To draw your rectangle or ellipse from the centre outwards, begin the selection by clicking and dragging in the image window. While the mouse button is down, press the Ctrl key. The Ctrl key must stay pressed until the selection is finished and the mouse button has been released.

 

Lasso

The Lasso is used by clicking and dragging in the image window, tracing out the shape to be selected. Releasing the mouse button completes the selection.

The Lasso tool is extremely difficult to use for fine detailed selections. It is a useful tool for creating masks.

Magic Wand

The Magic Wand makes selections based on specifying a particular pixel in the image. The pixels directly adjoining the first pixel are included in the selection if their colours are sufficiently close to the colour of the first pixel.  This process continues until there is a colour change in the pixels. 

Double clicking the magic wand brings up its options.

By varying the threshold setting you can control how close the colour matching to the first pixel must be.

Bezier Path

The Bezier Path selection tool is very powerful. It is the only selection tool that allows the interactive adjustment of the selections it makes. The Bezier Path tool works by placing control points on an image. These are initially connected by straight-line path segments; however, using the control handles  hidden inside each control point, the path segments can be made to curve wherever you want them.

Intelligent Scissors

The intelligent scissors work by following, as closely as possible, constant-valued colour contours in the image between subsequent points clicked by the mouse.  It is not of great value.

Selection Tool Options

Anti-aliasing

Anti-aliasing works to smooth 'jaggy' edges on a sloping selection. This removes the 'staircase' effect of the pixels.

Feathering

Feathering softens the edge of a selection, not just the sloping edges.