I2C - from http://en.wikipedia.org/wiki/I²C
I²C (Inter-Integrated Circuit) is a multi-master serial computer bus invented by Philips that is used to attach
low-speed peripherals to a motherboard, embedded system, or cellphone.
Open Source - from http://en.wikipedia.org/wiki/Open_source
Open source is an approach to the design, development, and distribution of software, offering practical
accessibility to a software's source code. See also http://en.wikipedia.org/wiki/Open_Source_Definition
Open Source Hardware - from http://en.wikipedia.org/wiki/Open_source_hardware
Open source hardware refers to computer and electronic hardware that is designed in the same fashion
as free and open source software (FOSS).
PCB - from http://en.wikipedia.org/wiki/Printed_circuit_board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate.
Plug - see JeePlug for more details
A "plug" is a small board which follows the connection convention of 6-pin ports. Some plugs are used by themselves, other plugs allow daisy-chaining (using the I2C bus).
There are also some small daughterboards, connecting to either 2 or or 4 ports.
Shield - from http://www.arduino.cc/en/Main/ArduinoShields
Shields are boards that can be plugged on top of the Arduino PCB extending its capabilities. The different shields follow the same philosophy as the original toolkit: they are easy to mount, and cheap to produce
Sketch - from http://www.arduino.cc/en/Tutorial/Sketch
A sketch is the name that Arduino uses for a program. It's the unit of code that is uploaded to and run on an Arduino board.