This is one indication of a digital SLR's durability, and it's measured as the number of exposures, or 'cycles' the shutter is designed to be able to withstand during its life. (You don't usually see this figure quoted for other camera types.)
Makers won't usually bother quoting a figure for basic or beginner-orientated cameras, but shutter life is often used as a selling point for more advanced or professional models.
A few years back, 100,000 cycles was regarded as the standard for professional cameras, but this has crept up and Nikon, for example, now quotes 150,000 cycles for its D7000 model, while its high-speed D3s pro SLR has a shutter rated at 300,000 cycles.
This sounds a lot, but it's not a warranty of any kind, just an indication of how long the designers expect the shutter mechanism to last - and it may go on much longer than that.
Amateur photographers are unlikely to wear out their cameras' shutters before the camera itself becomes obsolete and is replaced. If you do carry out any back-of-the-envelope calculations with your camera's shutter life, do remember that for every picture you save, you might take many more which you discard, and they all count towards the total shutter life.
