In a previous post, I described how to make basic selections, and more complex ones by combining areas by addition and subtraction.
Besides that, GIMP provides advanced tools to improve selection, which can achieve non-visible contours that will give your work a professional look.
Access to selection tools and their modification is centred in the generic tool window, with specific icons for each selection mode. In the below image, you may see the following icons:
1 - Rectangular selection
2 - Elliptic selection
3 - Free-hand selection ("lasso")
4 - Adjacent colour selection ("magic wand")
5 - Selection by colour
6 - Cutout selection (crop)
7 - Foreground selection
8 - Paths
Icons (6) and (7) are "special", and they will be presented in a separate post.
Some further icons in the lower section (9) complement the keyboard shortcuts described in the previoous post: Individual selection, addition, subtraction and intersection.
From that point, you may tick checkbox (10) if you do not want your selection to have a "clean" contour (by clicking this box, you'll get the chance to define an influende radius), bu tinstead you will obtain a progressive fading to contents in the lower layer.
In square or rectangular selections, you can also check (11) to produce rounded corners (useful to add some vignetting or washed look to pictures).
Checkbox (12) produces selection (either rectangular or elliptic) from the central point - we could already do this with keyboard shortcuts.
In case of rectangular or elliptic selection, a form-factor can be fixed prior to the selection, independently of the size. Or rather, selection dimensions can be defined by numeric input, fixing (if needed) selection width or height, from the drop-down menu.
With input fields (15), you may fix the selection origin,taking into account that coordinates (0,0) refer to the upper left pixel.
Finally, with checkbox (16) a grid can be overlay to the selected area. For photographers, there is a rule-of-thirds option, quite useful when defining image crops...
Depending on the active selection tool, you might find even further options available. For example, contour softening (17), available in all selection modes except for rectangular one, or the option to include transparent areas (information from alpha channel) in the selection (19).
A last point, option (20) for "combined sampling", uses the visible information of the selected pixels, independently from the layer they are in. If you do not tick this option, you might be selecting nothing from the active layer...