The kit comes with 8 red LEDs, a preprogrammed microcontroller, a serial port connector and a AA battery case.
If you buy the kit from the adafruit shop, the preprogrammed code just blinks the LEDs in order. Other stores often have a preprogrammed image already in it, for example the Make store has "MakeZine!" as seen on the front page.
For some reason, a batch of microcontroller chips (datecode 0627 ?)have been acting odd and shut themselves off after a few seconds. The problem seems to be that the RESET pin is being wonky, to fix this take a resistor (anywhere between 2Kohm and 20Kohm) and solder it to the top two pins of the microcontroller so that the RESET pin (pin #1) is connected through the resistor to the POWER pin (pin #20)
If this doesnt work, refer to the next question in the FAQ!