Limit Selection to Visible

When you have the display mode in Wireframe (Z) you are able to select vertices, edges, or faces that are in front of the model, as well as those that are behind it. This is not possible with the Solid display, which only lets you interact with the parts of the model that are facing you.

In Edit Mode, when a mode other than the Wireframe display is active, right next to the icons on the 3D View header for selecting vertices, edges, or faces is an icon with a little cube that has marked vertices, with a back side that is not visible. If that option is turned on, you’ll only see the parts of the model that are facing you, but if you disable it, you’ll be able to see any of the elements behind the model and you’ll also be able to select them. Sometimes, you want to select a large portion of an object and if the model has holes and cavities in it, is useful to be able to select all the vertices at once; however, most of the time you probably will only want to select elements that are visible and in front of the model, so that you don’t accidentally select something behind it.