Wiring is an open source electronics prototyping platform composed of a programming environment (IDE), an electronics prototyping board, documentation thoughtfully created with designers and artists in mind and a community where experts, intermediate and beginners from around the world share ideas, knowledge and their collective experience.
The Wiring i/o board is a small, cheap standalone computer with many connection capabilities. It can be easily programmed in a variant of the Processing language, with a similar programming environment. The board can control all kinds of sensors and actuators. Sensors allow the board to acquire information from the surrounding environment (temperature sensors, light sensors, distance sensors, etc). Actuators are devices that allow the board to create changes in the physical world (lights, motors, heating devices, etc). It can also interact easily with other devices and computers, such as your PC/Mac, GPS receivers, barcode readers.
Wiring is an open project initiated by Hernando Barragán (University of Los Andes, Architecture and Design School). It builds on Processing, the open source progra programming language and environment initiated by Ben Fry and Casey Reas. Arduino was inpired by Wiring and is basically the same thing. It is programmed in the Wiring language.