Before buying a home security system, you should understand what's out there in the way of alarms and how they work. This will help ensure you get what you need and don't get sucked into buying a whole bunch of fancy doo-dads that you'll never even bother activating.
According to The Idiot's Guide to Home Security, there are 4 basic levels of burglar alarms: simple premise alarms, local alarm systems, telephone dialar systems, and continually monitored security systems.
A simple premises alarm is something like a smoke detector or a battery-operated door wedge alarm, an alarm with a single function. These are usually fairly cheap.
With local alarm systems, a siren sounds inside and outside the house when the alarm is tripped. Though these may scare away burglars, they don't notify the police, so if nobody is around to hear them, the burglar might just wait until it goes off and try again.
Telephone dialer systems are programmed to call up to four numbers with a preprogrammed help message when the alarm is tripped. Panic buttons fall under this category of alarms.
The most effective and most expensive types of burglar alarms are continually monitored security systems. With these alarms, when the trigger is tripped, your security monitoring company is alerted. They will in turn call the police or fire department; some even send a security officer to your house.
For more information you may want to visit the following sites:
Burglar Alarm Basics
BBC: Burglar Alarms
How Burglar Alarms Work
And browse home security equipment: X10 Wireless Accessories
a shop that sells gear like this "high end video surveillance camera with ultimate zoom
" or this cheaper "Tiny Wireless Camera, See & Record Everything
".