Area selection is the method by which you can select more than one thing at the same time.  Area selection happens when you do a “drag” (LMB down, move the mouse, LMB up).  It comes in 3 flavors: Window Selection, Freeform Selection, and Paint Selection

Window Selection

The default is “Window Selection”.  You simply draw a rectangle by two points around things in your model to determine what is selected.

Window Select offers two different interactions:

Upper-left to lower-right.  In this usage, only things that are completely inside the rectangle will be selected:

Upper-right to lower-left.  In this usage, everything which crosses the rectangle gets selected:

Notice how much more was selected this way.  The yellow rectangle is a visual clue that you are doing a “crossing” selection, instead of the orange “inclusion” selection preview.

Freeform Selection

If you have your area select set to “Freeform Selection”, you can draw a less constrained shape to determine what is selected, instead of a rectangle.

Freeform Selection also has crossing and inclusion selection options.  Drawing your shape clockwise is an inclusion selection:

While counter-clockwise is a crossing selection:

Paint Selection

This kind of area selection allows you to select just by dragging the cursor over things you want to select. Paint Selection, by its nature, is always a “crossing” selection – anything the path of the cursor selects will be selected.  There is no “inclusion” selection with Paint Selection.