NFC or Near Field Communication is a communication protocol that enable two devices to establish communication by bringing them within a few cm of each other, typically when used in home automation one of these is a passive device powered by electromagnetic induction when in proximity to a powered device.
NFC Reader and NFC Ring
I have an NFC reader connected to a Spark Core that also displays the current home/away/bed status of the house and I can use my NFC Ring, a keyfob tag or card tag to manually set the status just be putting my hand over it. The NFC ring has two tags in it, one on each side so I have one set to toggle between home/away modes and one to set bed mode.
NFC Sticker Tags
This is something that was a bit of a failed experiment. I stuck some Mifare Ultralight and NTAG203 NFC tags stuck around the house, the idea being that when I put my phone down in a particular spot it can be used to trigger events or form part of the presence detection system. In practice this is a bit fussy, making sure the phone is unlocked when put down and put down in a fairly precise position is not very user friendly. I still have one in my car phone holder to set the phone into car mode, turn WiFi off etc. and on the Qi chargers I have on my desk at home and at work.