in photoshop, if u select the crop tool, leave the parameters on top EMPTY, crop ur image (drag the "rectangle" and double click inside or press enter), u will get a 100% crop.
in other words, ur image would look the SAME if u overlapped it over the original image in the correct spot.
hope this helps.
usually someone would display a 100% to show the details, sharpness or noise levels of a particular image taken by a particular body or lens, instead of posting the whole photo up which might take a while to download.