NFC (near field communication) tags are pretty much stickers that can hold a small amount of data. If your phone has NFC capability, it can read these stickers, which will cause the phone to perform whatever function programmed onto the stickers. It can open a web page, transfer a contact, or automate functions and settings..
The last part is the most interesting. You can buy NFC stickers pretty cheap and program them yourself to automate your phone as you see fit. If you use your phone on wifi at home, you've probably toggled wifi on and off thousands of times to save battery (I have). You can program an NFC tag to do that. When you get home and want to turn on wifi on your phone, you just hold the phone near or on the tag.
That's a simple example. People have programmed these things to change the phone state half a dozen ways with a single tag. Any setting or combination of settings and apps that can be launched is fair game (except the cell radio I think; there's one thing that doesn't work but I forget which). Instead of turning on Bluetooth, muting my phone, shutting off wifi, and opening the music app, I have a tag in the car that does that for me when I touch the phone to it.
Edit: initially, I was worried that having NFC on all the time would drain the battery and lead to misreads. The NFC radio, however, is on only when the phone is unlocked and the screen is on. There hasn't been a noticeable drop in battery performance or problems with the phone tagging something I didn't want.
There are different types of tags and some have issues with compatibility. I went with the tags and apps I describe above because they've been shown to work. Other types will definitely work, just go with ones that people confirm to be compatible with the ICS version you're running (4.0.2 in my case).
Facebook
Twitter
Instagram