Wireless receivers are add-ons devices which can be used with hardwired security panels (such as the Honeywell Vista series) enabling them to work with wireless devices. After the wireless receiver is installed, the hardwired system can now use the wide range of wireless sensors as an element of their setup.
Wireless receivers are relatively simple and easy to install. The main reason to consider wireless sensors is for ease of installation. While hardwired sensors such as motion detectors are much lower in cost, getting the wires to the device may be too difficult. This is where a wireless device shines. Rather than positioning the device (ex. motion detector) for ease of wiring, wireless allows you to position the device for best protection.
Many homeowners who have an existing hardwired alarm system may need to add protection because of remodeling or just the desire to enhance their defense. Adding a wireless receiver may be the way to go to make expansion devices easier to install as DIY.
There are "combo" keypads which are alarm keypads, but also contain an integrated wireless receiver. Using Honeywell as an example, the 6160RF keypad can be wired back to the Vista control panel, generally replacing the existing keypad, and now you have access to all of Honeywell's 5800 series wireless sensors - door/window transmitters, motion detectors, glassbreak detectors, smoke and/or CO detectors, even keyfobs.
Wireless receivers whether standalone or integrated with a keypad are however brand specific. In some cases they may be model specific as well. As mentioned above Honeywell has their 5800 series wireless devices, but they also have their new SiX series as well. The two are not compatible. The SiX series are encrypted devices using a completely different communication protocol. These are meant to work with Honeywell's Lyric system which is also backward compatible with the older 5800 series devices. Honeywell does not yet offer a standalone SiX series wireless receiver.
While we state that wireless receivers and matching wireless sensors are brand specific, there are exceptions. 2GIG uses the same frequencies and protocol as Honeywell's 5800 series. This means 2GIG wireless sensors can be used with any 5800 series wireless receiver. Same manufacturer such as Resolution make sensors specific to their own systems (Helix), but also make compatible sensors for others like Honeywell. Always check to make sure any wireless device you add says its compatible with the wireless receiver/series you are using.