All of the Resize options are actually only applicable when you have changed the image dimensions. If your dimensions are consistant, then it will always do a simple resize.
When the proportions are changed:
1) Stretch will be similar to what an image does naturally when dimensions are changed in html
2) Fit to box will find the largest proportional image that will fit in the width and height specified, center it in the image and fill the rest with the fill color (except if the image uses transparency, then transparent becomes the default fill color)
3) Crop from center will resize as much as it can to fill the entire area, then crop the leftover. Unlike fit to box it won't fit the whole image and fill, it will make it larger so no fill is needed and crop the remaining.
4) Proportion to box will find the largest image that will fit in that area, just like fit to box, but instead of filling with a color to make the final image the exact width and height specified it will change the html to reflect the new image dimensions so that no fill is needed.