The Arduino is a programable AVR micro-controller. You can re-program it with a USB cable and a computer as many times as you want, changing whatever you want. It is based on the Atmega168 micro-controller chip which is very feature-rich.
It only costs about $25 for a complete Arduino, and replacement chips are only $4, so once you get a program the way you want, you can take the chip out and install it into your project... then put a new one your Arduino and keep going!
Once you get the arduino, you can download the free programming software for your computer, then figure out what you want to do with your arduino. My first project (like most people) was to blink an led. The Arduino programming language is easy to learn and there isn't a whole lot to remember, which is excellent if this is your first experience programming anything (like me).
this is the Arduino powering some LED's via an RC receiver/transmitter.