Most thumbnail-generation solutions will shrink the original down while maintaining aspect ratio.
Usually you specify the maximum height and width of the thumbnail, e.g. 150 x 200
However, if your original image’s aspect ratio is different to the maximum thumbnail dimensions, you will end up with dead space vertically or horizontally (shown in green in the illustration). This can be quite an eyesore when displaying thumbnails in a grid.
I’ve got an algorithm that will automatically crop the image either horizontally or vertically to then match the thumbnail aspect ratio, so you end up with the thumbnails all being the same size even though they may be coming from originals of wildly different aspect ratio.