Proof is a photographic term describing a print made for evaluation purposes.
Generally smaller than the final print and made with less care in the printing process,
proofs are used to select which photographs will undergo the final printing process
(which is expensive in both time and money). Digital photography is no different,
if anything proofs are more important, as it is possible to take thousands of
pictures a day, something few film photographers have the means to do. Digital images
are generally cropped, color corrected, and sharpened befored being printed or
published on the web.
In other words, the final image may be cropped on the subject, will have far
more detail than shown in the proof, and will look significantly better. To
give you an idea as to size, I generally proof between 450 and 640 pixel wide
images, while the full size of the images from my newest camera (Canon 20D)
are 3504 x 2336 pixels wide...so a 450x300 proof only has one-sixtieth the
number of pixels!
An example...here is the proof of an image at 450x300:
Here is a portion of the original image showing the amount of detail present:
Here is that same portion with sharpening applied: