RFID (radio-frequency identification) is the most common way access cards and dongles are used to identify you or another object. There are a couple basics to keep in mind when you are considering incorporating RFID into your Arduino project.
RFID readers and tags either work at 125 kHz or 13.56 MHz and you can’t mix the two. So if you are tied to a particular reader or a particular tag, make sure your other hardware is on the same frequency.
There are two popular RFID reader manufacturers that both communicate over Serial to the Arduino.
Innovations produces a series of readers that range from not having an antenna (ID-2) to have two different built-in antenna strengths (the weaker ID-12 and the stronger ID-20). Bildr has a nice tutorial with some sample code.
Parallax also makes a reader. You can find a tutorial with sample code on the Arduino Playground.