The Arduino is a microprocessor board that uses the Atmel Mega168 (older versions used the Atmel Mega8). The controller board features input/output pins, the ability to use battery or wall adapter power, and can be programmed via 9 pin serial, USB, or bluetooth, depending on the hardware version.

The main Arduino website: http://www.arduino.cc

The Arduino is programmed in C via an integrated programming environment that includes the program editor, the compiler, and the programmer. The Arduino programming environment runs on Linux, MacOS X, and Windows.

The Arduino is an open source project, and the programming environment, schematics, bootloader, and source code are freely available to anyone. Unlike other programming environments that charge hefty fees for programming environments, or give away "lite" versions that are limited in capabilities, Arduino software can be downloaded by anyone and used for free. This makes the Arduino uniquely suited for beginners or those on a budget. A 9 pin serial version of the Arduino can be purchased for less than $20.

Purchase an Arduinio USB board from Zagros Robotics. ($34.95) This Arduino board uses a standard USB connector for both programming and power to the board. When you install the board in your robot, it uses battery power.
Purchase a Bluetooth board from Zagros Robotics. ($145.95) This Arduino board uses a Bluetooth connection for programming. A Bluetooth adaptor installed on your PC is required for programming. This board will allow you to control your robot with a Bluetooth device such as a cell phone.
Purchase a 9 pin serial board from NKC Electronics. ($16.99) This Arduino board uses a 9 pin serial connection for programming, and is a good choice for beginners. If things go horribly wrong and you blow it up, you are not out a whole lot of money! Your PC may not have a 9 pin serial connection...a lot of new PC's don't. In that case, give a USB to Serial adaptor a try. I use this one from Keyspan, and it works like a champ on my Ubuntu box, so the chances of it working for MacOS X or Windows are nearly 100%.

Once you have purchased an Arduino board, see the ROBOMO wiki how to on getting it connected to your PC.

Below is an example of the typical "hello world" program, which blinks an LED. This screenshot shows the simplicity of the programming environment.