Freeform option lets you resize your selection with no regards for it’s original proportion and ratio.  

With Freeform, your content can result in being squashed and stretched depending on the direction you move the controllers of your content’s bounding box.  

The Uniform transformation option lets you resize your selection with regards to it’s proportion and ratio. For instance, if you increase the height of your selection, the width of the content will automatically be adjusted preserving your selection’s original ratio.