Potatoes can be separated by basic types (round white, russet, round red, or long white), or more usefully by waxy versus non-waxy. Waxy potatoes are relatively high in sugar and low in starch. When cooking, they will hold their shape. Waxy potatoes are best for something like roasted potatoes or potato salad where the shape is important. Examples are Red Bliss and fingerling varieties. Non-waxy potatoes are the opposite of waxy, high in starch and lower in sugar. When cooking, they'll lose their shape and the starch granules will swell. Non-waxy potatoes are best for mashing, baking, and frying; for French fries, the low sugar content will allow them to cook thoroughly before browning too darkly or burning. Examples are Russet, yams, and other sweet potatoes. Lastly, there are a few all-purpose varieties that are really good for anything: Yukon gold, red gold, purple majesty, or other red or blue varieties! As mentioned above, Russet, Yukon gold, or (now that you know) any non-waxy or all-purpose variety is best for soup since it will be blended and have lots of liquid to absorb.