There are two main types of cherries out there: sweet cherries and tart cherries. Sweet cherries, or dark cherries, have a much sweeter flavor and are the type of cherries that you’ll see sold in bulk for eating out of hand. Bing, rainier and sweetheart are just of the more common varieties of sweet cherries out there. Tart cherries have a sharp, tart flavor and are not typically eaten as is. Instead, they are used for juice and called called for in recipes for everything from pies to preserves. This is because that tartness can be balanced with additional sugar to bring out the cherry flavor in a fruit that isn’t quite as delicious on its own as a sweet cherry.
Some recipes will call specially for tart cherries and others will not specify which type of cherry should be used. When baking with cherries, most of the time you can use either sweet cherries or tart cherries as long as you pay attention to the sugar content of the recipe so that your baked goods end up balanced. A recipe that specifically calls for tart cherries may have extra sugar added to it. A recipe that specifically calls for sweet cherries may need some extra sugar if you are using sweet cherries. Some sweet cherries, like rainier cherries, can lose a little bit of their texture when baked. Just as when baking with apples, however, there are people who like all different textures in their finished fruit (and “mushy” to some can be “very tender” to others) as long as a great flavor comes through.
I personally prefer baking with sweet cherries and will always specify if I go out of my way to use tart cherries instead. I like their sweetness, their color and their juicy cherry flavor, so that is what I usually default to when baking things like cherry pie.