A photograph can easily be ruined when handshake or vibration is introduced during long exposure shots. The Image Stabilizer (IS) counters such problems by optically correcting such shakes with gyro sensors and lens groups that move in relation to the shakes, thus minimising or even eliminating minute vibrations from the image.
A general rule-of-thumb to overcome such vibrations would be to set the shutter speed equal to or faster than the reciprocal of the lens' focal length (e.g. 1/60s for a 50 mm lens). IS lenses can improve on this rule by up to three stops. That is, the same 50 mm lens could be used at 1/8s.
Canon has released several versions of the IS system. The original, first used in 1995's 75-300mm f/4-5.6 IS USM, takes approximately one second to stabilize, provides approximately two stops of stability, is not suitable for use on a tripod (if it cannot detect any motion, it may introduce unwanted motion of its own), and should not be used while panning. The next advance was released with the 300mm f/4L IS USM in 1997 and adds IS mode 2, which is for panning. Mode 2 detects whether panning is taking place horizontally or vertically, and only compensates for vibration in the plane perpendicular to the plan of panning. In 1999, with the release of the IS super-telephoto lenses (300mm f/2.8L IS USM through 600mm f/4L IS USM), tripod detection was added, so that the lens could be used on a tripod with IS enabled. The most recent advance, first found in the 70-200mm f/2.8L IS USM released in 2001, reduces startup time to approximately 0.5s and increases stabilization to three stops. Canon has not ported newer IS versions back to older IS lenses.
: The photos from our trip! Overhere! Feel free to use any of these additional letters to correct the spelling of words found in the above post: a-e-t-n-d-i-o-s-m-l-u-y-h-c