A "fruit pie" specifically designates a particular kind of junk food, made by Hostess and many other companies (particularly I've noticed some regional bakeries make them). It is essentially a small pocket of dough, loaded up with a fruit filling and covered in sugar. They are also loaded with preservatives, and exceedingly fatty (I once joked that, ounce for ounce, they were worse for you than cigarettes). They are also pretty cheap (usually on the order of 50 US cents); you can get them pretty much anywhere (convenience stores, gas stations, bakery outlets, you name it).
Contrast with a regular pie, which may or may not have a fruit filling. The name "fruit pie", I think, expresses the generic qualities of these pies. You don't call a regular cherry pie a "cherry fruit pie", but that is not an uncommon term when discussing these heart attack inducing treats.
Common flavors include apple, cherry, blueberry, and lemon. Hostess was probably the first to come out with them, but I was unable to find anything one way or another through Google.
This terminology was probably (partially) spread by a show on Fox called King of the Hill. In it, there is a strange, fat little child named Bobby, with a serious obsession for fruit pies.
Here are some "interesting" nutritional facts about Hostess cherry fruit pies:
Calories from fat: 200
Total fat: 22 grams (34% daily value)
Total saturated fat: 11 grams (55% daily value)
Sodium: 470 mg (20% daily value)
(Taken from http://home.att.net/~product_access_project/HostessCherryFruitPie.html)
So, while these things are a great snack, don't eat them too often, OK? I would hate to see a dramatic noder reduction due to death by obesity and stroke. Personally, I get sick of them if I eat more than one every 2 weeks, which all in all seems like a good situation.