Sauté vs. sweat

One of the ways where the level of heat makes a difference is when sauté or sweat vegetables. Your goal is different in the two techniques. It is largely a matter of being aware of the amount of heat you're using.

When you sauté vegetables, you use a higher level of heat. Some form of fat - oil, lard, butter, or some combination thereof - is used to "hold" heat to transfer to the vegetables. The flat surface of the pan cannot touch all the nooks and grooves of the vegetables (imagine the shape of a broccoli floret), thus, the fat that coats the veggies carries heat to cook them. The point of using high heat is to cook quickly, and keep as much of the flavor in the vegetables as possible. After sautéing, the veggies are pretty much a finished product. You may toss them in some kind of sauce or other ingredients, but not much else.

But sometimes, you actually want to sweat your veggies. The goal of sweating is to draw liquid out of the vegetables, pulling out their flavor. This is done over lower heat, with a little fat, and a little salt added. The salt is essential, as it is hygroscopic - it attracts water. Thus, the moisture in the veggies is drawn out like a magnet by the salt. The low heat is important, as higher heat will seal the moisture in. Unlike a sauté, sweating is usually an intermediary step. For example, you might want to be drawing the flavor out of veggies to make a broth for a soup.