HDMI Video Selectors route video from one of several sources to one (or more) destinations. For instance, if you had both a HD cable box and a DVD player with HDMI outputs, you could use one of these devices to select which source device connected to your TV (sink device).
Some switches have automatic switching, as well as manual. They will automatically switch to the last source device turned on.
Some switches have multiple outputs. Unless stated otherwise, multiple outputs will show the same source device at any given time. A few devices are called "matrix switchers" and they can show any source device on any output (sink device).