These images are taken with a pair of cameras mounted at the approximate spacing of the human eyes. When they are viewed correctly the brain combines the two images and produces a 3 dimensional recreation of the original subject.
To view them usually requires a bit of practice to start with but once the knack has been achieved it is easy to repeat. Sit squarely to the computer at a comfortable viewing distance and stare at the images trying not to focus on them directly but rather focus beyond them. Some viewers find it easier to use the "assist spots" to start with. Eventually you should start to see three images with the central image appearing 3 dimensional. ( If you initially find it difficult it is worth persevering as the result is quite dramatic when you do achieve success ).
Some people see the correct effect more easily with the images reversed ( this is known as cross-eyed viewing as distinct from parallel viewing ).
The images are presented in both formats with the parallel set uppermost.( Once you can see the correct effect in either format the other format should appear reversed ie. foreground objects will appear submerged into the background).
Some interesting information regarding stereo viewing can be found here ( The "How to see 3D" link is helpful ).