From BlenderWiki

Local or Global View

Description

When in Local view, only the selected objects are displayed, which can make editing easier in complex scenes. To enter local view, first select the objects you want, and then use the View » Local View menu entry. Use the View » Global View menu entry to go back to global view. / NumPad toggles between both views.

Note that the layer and lock buttons on the 3D View header disappear while in local view.

Examples

Global view.

Local view.

In (Global view), the layer buttons are visible and the green cube is selected. (Local view) shows that the cube has been focused and centered in the 3D View and the layer buttons have disappeared. If a scene has thousands of objects visible, this feature can potentially speed interactivity up because only one or a few objects remain visible.