How are the many potato varieties different from one another?
Although all vegetables vary by size and freshness, most markets carry only a single variety. Broccoli is broccoli, carrots are carrots. Even when there are several varieties (as with heirloom tomatoes), most can be used interchangeably in recipes. Yes, one tomato might look a bit different or be a bit sweeter than another, but they all will taste fine in salads.
With potatoes, this is not the case. Make French fries with Red Bliss potatoes and the fries will be greasy and heavy. Use russets in salad or corn chowder and they will fall apart in a soggy mess.
The fact that dozens of potato varieties are grown in this country makes the question of which potato is best for a specific recipe even more confusing. At any time you may see as many as five or six kinds of potatoes in your supermarket. Go to a farmers' market and you may see a dozen varieties. Some potatoes are sold by varietal name (such as Red Bliss or Yukon Gold), others by generic name (all-purpose, baking, etc.).
To make sense of this confusion, it is helpful to group potatoes into three major categories based on their ratio of solids (mostly starch) to water. The categories are high-starch/low-moisture potatoes, medium-starch potatoes, and low-starch/high-moisture potatoes.
High-starch/low-moisture potatoes, such as russets or Idahos, generally lose their shape when simmered in soups or stews. Because they have so little moisture, they tend to soak up liquid as they cook and eventually implode. In some cases, as when you want the potatoes to thicken a soup, this can be desirable. Medium-starch potatoes, such as Yukon Golds and Yellow Finns, do a better job of holding their shape but share many traits in common with high-starch potatoes. Low-starch/high-moisture potatoes hold their shape better than other potatoes when simmered. This category includes all red-skinned potatoes, such as Red Bliss and Red Creamer, as well as freshly dug potatoes, which are often labeled "new" potatoes. Low-starch potatoes should be selected when potatoes are to hold their shape, as in Rustic Potato-Leek Soup.