In photography forums and review you will often see the term "100% crop". What does it mean?
A 100% crop is a crop, not resized, from the full size image. If you want too see a detail of a photo at full resolution, you have two possibilities: you can show the entire photo at full resolution (but it is not recommended for internet, since it would take some time to download), otherwise you can show just a small portion of the photo. In other words, to reduce the image size for internet, you crop a small portion of the image, instead of resizing the entire image to 800 or 720px.
For example, this is a photo taken with the Canon 5D (that has a resolution of 4368x2912 pixels), resized to 1200x800 pixels, for web viewing.
And this is a 100% crop from this image:
I have cropped just a small portion of the photo, but the detail has not been resized, so you can actually see the detail at full resolution.
To make another example of 100% crop:
It does not matter if the crop measures 630x432px, 255x333px or another measure; what does it really matters is that you take the crop from the full size photo, without resizing it to lower resolution. (i.e. you must not reduce the image size with Image>Image Size).
For example, this is not a 100% crop:
...because the file had been resized to 2000x3000 pixels before cropping, so you no longer see the detail at full resolution.
The 100% crops are very useful to evaluate the image quality of a lens or a camera, because they allows to see the detail at full size, as if you were looking at a 8, 10 or 12 megapixel image, but without the need to download large files.