The principle behind Arduino is an ability to control physical objects using your computer. The Arduino provides the interface between your computer and the object via the IDE, and will fire relays, LEDs or actuators based on provided commands. For people looking for an introduction to the world of Arduino, one of the simplest projects is to use Arduino to provide computer control over a locked door. The following projects are different variations on this theme and should provide some good resources to get you up and running with your first simple Arduino project.
This project uses Arduino in conjunction with a ID-20 RFID reader to provide an RFID equipped door lock for a front door or garage. The Arduino receives serial data from the tag reader and fires an LED and a relay controlling the lock when the correct RFID tag is used. This is a relatively simple Arduino project that is well suited for beginners, and can be really useful in allowing you to open your door while your hands are full. The system does require an electric door lock that can be controlled by the Arduino.
Garage door openers are a popular beginner Arduino project, and this one combines the traditional garage door relay with a web server capability. The Arduino receives serial commands from a php script that is running on a home web server. The website running on the server is accessible from a mobile device on the network, and updates with the status of the door, whether it is up or down in addition to controlling the door's movement.
The keypad garage door opener, despite at first glance offering less sophisticate functionality than the RFID or mobile solution, is perhaps more difficult to implement. This is a result of wiring the older technology of the keypad into the Arduino, and correctly recognizing the different key-presses, as well as the correct combination of key presses to trigger the lock relay. As a result it seems slightly less desirable than the previous projects, but perhaps it is a good challenge for creating more manual interfaces into Arduino.
This project adds an Arduino based locking mechanism to a small wooden box, creating a smart secure lockbox. The unique aspect of this project is that it uses an Optical Finger Navigation (OFN) sensor to detect the movements of a finger. The sensor looks for a secret movement pattern of your finger on the sensor to provide the authorization for the locking mechanism. A single arm servo powers the locking mechanism on the box.
This is not a door lock project for Arduino that you necessarily want to try out per se, but an interesting finding made by a group of black-hat Arduino hackers. This group used Arduino to create a device that acts as a master key for hotel room locks that will open one of the more prevalently used brands of locks used at many hotel chains. A group later pared down the components of the device to fit inside a dry erase marker. This is probably not an Arduino project you want to try, as it could land you in a lot of trouble, but it is an interesting illustration of the versatility of the Arduino platform at controlling physical objects.
These door lock projects comprise some of the more simple applications of Arduino in controlling real world things. If you are looking to take the plunge into Arduino, perhaps a door that can be opened with your mobile phone is a great introduction.