Idaho and Russet potatoes are the same type of potatoes. These names are used interchangeably.
There are many different types of potatoes. Each of these imparts a different flavor, color and texture. Idaho potatoes are quite starchy, so they tend to be less flavorful than others. Also, they are larger and end up being more powdery when they are cooked. They are particularly great for making smooth mashed potatoes. Yukon gold potatoes have more flavor to them, though they also are a starchy type of potato. Fingerlings are more waxy, so they are not as good to use in mashed potatoes, and they have a skin that is thin and tight.