Business Model Canvas is a strategic management and lean startup template for developing new or documenting existing business models. It is a visual chart with elements describing a firm's or product's value proposition, infrastructure, customers, and finances It assists firms in aligning their activities by illustrating potential trade-offs.
Here's an illustration of the canvas and its components.
More information can be gained at this discussion on its evolution as a planning tool.
A lot more information and guidance is available at Strategyzer.com and Canvanizer.com - the latter has the capability for developing a canvas online and sharing it with others, such as your SCORE mentor!