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APS-C
Advanced Photo System type-C (APS-C) is a type of sensor used in digital Single Lens Reflex cameras, also called DSLRs. This sensor has a smaller sensor dimensions than that of the 36 mm x 24 mm conventional 35 mm sensor SLRs, causing a 1.x multiplier to focal point distance when comparing lenses made for conventional 35 mm film.
Several manufacturers now make lenses specifically engineered for APS-C cameras. These include Canon's EF-S line, Nikon's DX, Pentax's DA line, and Sigma's DC line.
Multiplier factors
The most common multiplier ratios (in order of announcements, discountinued marked with *):
1.6x - Canon EOS 30D, Canon EOS 20Da, Canon EOS Digital Rebel XT, Canon EOS 20D*, Canon EOS Digital Rebel*, Canon EOS 10D*, Canon EOS D60*, Canon EOS D30*
1.5x - all Nikon DSLR, all Fuji DSLR (Fuji uses Nikon bodies), Sony DSLR-A100 (Sony bought Konica Minolta), Konica Minolta Maxxum 5D*, Konica Minolta Maxxum 7D*
1.3x - Canon EOS-1D Mark II N, Canon EOS-1D Mark II*, Canon EOS-1D*
Lens differences, incompatibilities
Lenses with 1.6x multiplier - also called crop - such as the Canon EF-S line, place the rear of the lens closer to the camera's sensor (referred to as Short Back Focus). This has several benefits, including increasing the focal range. It also has downsides, in that the lenses aren't compatible with camera bodies that use a full-size mirror, as it would damage the mirror, and thus the lenses can't be used on other digital or analog bodies.
Cameras capable of using short-back lenses include the Canon EOS 300D and 350D - also known as Digital Rebel and Digital Rebel XT - as well as the 20D and its latest successor, the 30D. These cameras support conventional 35 mm lenses as well.
Links
Focal lenght multiplier: http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_Multiplier_01.htm
Digital crop explained: http://www.millhouse.nl/digitalcropfactorframe.html
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