Door and window protection is what makes up a home security system perimeter defense.  There are different types and styles of both wired and wireless devices designed to detect when an intruder opens a protected door or window.  Wireless versions are basically wired magnetic contacts with an integrated transmitter to signal the main security panel a change in status has occurred (door opened or closed, battery low). When selecting wireless sensors be sure they are compatible with your base system (e.g. Honeywell,Helix, DSC, 2GIG).

Magnetic contacts are the most common sensing devices for home security systems. They are cheap and reliable. Magnetic contacts are two-part devices used to safeguard windows and doors. One part of the contact is a switch installed in the window or door jam; the other contains a magnet and is placed in the window or door itself. Any noticeable shift in contact with the switch results in an alarm.  These can be home-run wired back to the main control panel, daisy-chained with other door/window contacts or part of a wireless transmitter combo.  You can select surface mount devices or recessed mount meaning they are hidden from view and both come in wired and wireless versions.  The fastest wireless install is a surface mount transmitter which takes under five minutes per opening.  Recessed wireless is a little more labor intensive (requires drilling) but aesthetically far superior.