Raspberry Pi is a powerful mini computer that runs Linux, and can be set to automatically interface complex tasks.
Download and extract the NOOBS zip file here: http://www.raspberrypi.org/downloads/
Next, download the ApplePi Baker software here: http://www.tweaking4all.com/hardware/raspberry-pi/macosx-apple-pi-baker/
Plug in an SD card (at least 8 gigabytes), and open up the ApplePi Baker.
Select the SD Card and click the "Prep for NOOBS" button.
Next, open the extracted NOOBS file and copy everything onto the now formatted SD Card.
Unplug the SD Card and insert it into the Raspberry Pi.
Connect it to a power supply, Keyboard, mouse, and monitor through HDMI.
Once it boots for the first time, select Raspbian.
After it installs, plug in an ethernet cord and enable SSH through the raspi-config menu.
Once the terminal loads up, type in "pi" for the username and "raspberry for the password.
Next, Type in "ifconfig". and write down the inet addr (a bunch of numbers).
Type in "startx" to access the GUI.
If you did this right, you should now be able to use your Pi as a computer.
In addition, now that SSH is installed, you can access your computer from any terminal, simply by typing "ssh pi@"inet addr"".
Now, if you want, you could disconnect the keyboard, mouse, and HDMI cable, allowing you to run it "headless" from another computer.