3D cameras are actually two cameras that capture images from slightly different angles.
3D cameras are actually a rig of two or more cameras that aim to capture images at the same speeds and angles as the human brain. Both images are then projected back to a viewer, from slightly different angles, and require glasses with polarization or coloration to achieve 3D perception.
Binocular
Our brain perceives 3D images through binocular vision, by compiling two images from each eye, a phenomenon known as stereopsis. The images perceived from our eyeballs are roughly 3 inches apart, therefore some 3D cameras are designed with dual lenses about 3 inches apart. The 3D glasses form a shutter effect, alternating images seen by each eye, similar to how our brain handles this information.
Mirror Rigs
In certain types of shots, particularly closeups, 3D camera must be placed extremely close to one another, in which case mirror rigs are used. Mirror rigs are two cameras that operate out of the same lens, but spit the image inside the camera using tiny mirrors.
Distance
From same angles and speeds, it is extremely difficult or impossible to capture an image with perfect 3D perception. Filmmakers and directors write screenplays and direct movies with shots that triangulate geometric distances friendly to 3D cameras, that they know will end up well on the 3D screen. Careful shot planning is certainly helpful in all types of 3D imagery.
Software
Digital imaging software is uses advanced geometric algorithms to match images up in the correct relation to create a 3D image. Advanced software and its capabilities are a big part of 3D feature films, and because of 3D's reliance on software, most 3D cameras are digital.
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