Culinarily, potatoes tend to be classified as either waxy or starchy (sometimes called mealy), based on their moisture and starch content. Each type is good for some things and bad for others. Knowing the difference between them can help you choose the potato that will be most successful for a given application.

Waxy

Of the two varieties, waxy potatoes are higher in moisture content and lower in starch. This makes them great for applications where you want the potato to maintain its shape when cooked. Generally if you’re roasting, pan frying, sauteing or using in a soup, waxy potatoes are going to be the best candidates. Waxy potatoes generally have a thin, soft, smooth skin. Look for red potatoes, Yukon Gold, gold potatoes or fingerling potatoes.

One notable exception is that some waxy potatoes fluff up well for mashed potatoes. Yukon Gold and German Butterball are two waxy potatoes that are commonly used for mashed potatoes. Most waxy potatoes will turn gummy when mashed, though.

Starchy

So, logic follows that starchy potatoes would be the variety with lower moisture content and higher in starch. This makes them great for applications where you want fluffy potatoes that don’t need to hold their shape. Baked* potatoes (not roasted potatoes, mind you), mashed potatoes, french fries and purees are good uses for starchy potatoes. Starchy potatoes have thicker skins that are often fuzzy. Russet potatoes are the most commonly available starchy potato. 

* Side Fact: In the UK, baked potatoes are commonly called “Jacket Potatoes”. I assume that helps them with the whole baked potato/roasted potato dilemma.