When this item is selected, every polyhedron that is inside or partly inside the red selection rectangle, will be selected. Notice that only polyhedrons will be selected, however polyhedrons which belongs to entities, like a door, will also be selected, but not the entity itself.

When this item is selected, every entity that is inside or partly inside the red selection rectangle, will be selected. Notice that only entities will be selected, entities which have polyhedrons belonging to them will not be selected.

This mouse-mode combines the polyhedron and entity selections, also it will select any duplicators and bezier-patches.

To quickly create a cube with the default texture, select this icon, and drag in a map-view to size the new cube.

"Like a knife through butter." 

Mark this icon in the mouse-modes toolbox, when you want to cut polyhedrons in two by dragging a line through the polyhedrons.

Remember you can always undo your actions if you're not satisfied with the result.