Selection

Selection is a feature that allows to isolate a region for a variety of uses. Applicable to both, layers and masks. e.g. copy / cut, color fill / draw & erase within the selection etc.

Making and Modifying selections
Select menu - over on the top panel in general menu there is a Select menu drop-down. It contains various options in regard to selection control.
Area Select Tool allows to make a rectangular selection.
More on Area Select Tool here
Lasso Tool is a freehand selection making tool.
More on Lasso Tool here
Auto Select Tool allows to automatically select "similar" pixels. The criteria for pixel similarity can be set in Tool Options menu.
More on Auto Select Tool here

Use modifier keys Ctrl to Add to current selection and Alt to Subtract from current selection when using selection tools.

Additionally you may want to use Move Tool transformations to transform selections.
More on Move Tool here

Spree to selection - turn contents of spree layer into a selection by pressing Ctrl+B
note: content transparency will be transponded to a resulting selection.

From layer - contents of layer can be used for creating a selection. Content transparency will be transponded to a resulting selection. Mask presence, and layer opacity will be taken into account, when creating a selection from layer view.
Usage: select a layer, then:
Select->From Layer + (if you would like to add to selection), or
Select->From Layer - (if you would like to subtract from active selection)
Alternatively Ctrl+Click on a layer in layers tab, to create a new selection using layer.
note: in this case layer does not have to be active/selected, and a layer can be hidden.

Select content is a part of general Select menu drop-down.
Select->Select Content.
Alternatively press Ctrl+A
This will make an opaque selection of all content present on the selected layers (multiple layers can be selected).
note: mask content can be selected same way, but only one mask can be selected at a time.
Deselect / Toggle Selection
Select->Deselect to clear selection
Press Ctrl+D to toggle selection on and off.
note: this will not bring back a selection cleared by deselect method above, only the one that has been initially toggled.
Moving selection - selection can be moved by click & dragging on selected region. One of the selection tools must be in use.
Additionally, can use arrow keys on keyboard to nudge.
note: Area Select Tool has a special embedded feature - selection will not move past canvas boundary. This is especially useful for cropping.