I’ve got one in my wallet that I just wave near a sensor to open the gate into the parking garage and the door to my office building. I’ve also got an EZ-Tag on my windshield that pays the toll on the tollway for me?
In laymans terms, how do those things work?
A tip of the hat to Shag.
I’ll buy your next drink in Callahan’s Crosstime Saloon, amigo!
What an obscure reference 
There are two basic types of RF tags, powered and unpowered. The powered type, which is what is described on the how stuff works site, have a battery powered chip which receives radio waves and, when it gets a specific type of signal, sends back a coded radio wave to identify the number programmed into the card. The unpowered type works basically the same way, except that instead of using a battery, the incoming radio waves are used to charge a capacitor (a tiny electronic component that can store electrical energy). Once the capacitor has stored enough of a charge to operate the microchip, the chip switches on and functions pretty much the same way as the battery operated version.
The advantage of the unpowered (or more properly called beam powered, since they are powered from the incoming radio beam) is that they don’t have a battery to replace, so they last longer. The disadvantage is that because they have to rely on the power from the incoming radio beam, they don’t have as much power as a battery powered one, so they usually won’t have as good of a range.
A lot of the really thin wallet sized cards are beam powered.
Here’s a decent pic of what’s inside one of the cards:
The square-ish spiral on the outside is the antenna.