Wired or wireless. Security system monitoring notifications are typically sent over the phone. Thieves know this, too, and may opt to cut home phone landlines, rendering landline-based monitoring useless. Wireless (cellular) based notification bypasses this problem, but requires adding such cell service (which may be bundled into the monthly monitoring cost).
DIY Network - A Consumer's Guide to Home Security Wireless Alarm Systems
Honeywell - Smart house - Safe Home.