The Pebble is a basic set of sensors and devices that can be used to learn about programming the Arduino.
The basic Pebble design always includes a rotary encoder, a temp sensor, an LDR, an XBee form-factor socket and an LCD screen.
Pebble Version 1 was a shield usable with an Arduino Uno/Duemilanove (or compatible) form factor. This also had a few buttons for feedback.
Pebble Version 2 was an all-in-one device that included an ATmega328P on-board (Uno compatible), included a buck-inverter so that the device could be run off a wide variety of voltages, an RGB Led, and 2 open collector transistor outputs.
In each case the LCD screen is connected to the Pebble, usually by a ribbon cable.
Both versions were designed by Luke Weston.
The Version 1 design was predominantly created with through-hole components. It was created for use at the Arduino Mini-Conf at LCA2010.
The Version 2 design was rebuilt with predominantly surface mount components, with a few through-hole components provided unpopulated. It was created for use for use at the Arduino Mini-Conf at LCA2012.