Why is cooking a food item method called different things by what the item is, or what is the criteria?
On the Food network they boil potatoes, but they poach carrots.
They poach turkey, but they boil eggs.
They sauté' onions, but they fry eggs in the same pan.
Likewise, they fry hash browns, but they sauté' onions in the same pan before adding the potatoes.
As pointed out already, these terms have quite specific and different meanings but it's worth being aware that they often get used interchangeably a lot too, and therefore possibly incorrectly.
A lot of the specific terms will come from french cuisine (like saute) including all different names for exactly how you've sliced up your vegetables (julliene, brunoise etc). I believe this was so recipes could be written very precisely and therefore reproduced more acccurately.