Bitter Turnip fries

Here is an excellent recipe by the guys over at Cooking Issues (and also a follow up report) with almost everything you'd ever want to know about chips. That first recipe is supposedly good even after they've cooled off:

Our standard fries are good even when cold.

Another option is to cook a lot of chips but stop after their first frying and freeze them; when you want to eat them, simply put the frozen chips into the fryer for the second frying. This also has added benefits:

Freezing acts like partial dehydration. When the frozen fries are finished, they liberate water freely, leading to rapid dehydration and good crust formation with a porous interior. Pre-frozen fries are crunchier than fresh and stay crisper longer after they are fried, but they tend toward hollow fry.

Good Eats(Alton Brown) has some good descriptions of the various difference's between potatos. He has an episode dedicated to potatoes. Main thing is the starch content in them. He has an example of each of the three groups (High, Medium, Low), includign a brief description and sample application. He mentions mentions frying in the High Starch content group, specifically Russets. He then has another episode simply about frying, where he actually talks about French Fries and again mentions the Russet (High Starch).

While you can make fries out of different types of potatoes (some of the more "all purpose" potatoes), if you want a really good classic french fry, you're going to want to look for a Russet, with a similar starch content.

Here's a page that lists some of the more common varieties. Maybe you can look to see what you bought? The age will probably affect it. A younger potato will in fact be sweeter

So, to answer your specific questions. I'm betting you did in fact buy a potato. It's probably just a sweeter variety. If it doesn't say at the market where you bought it what kind they are, I'd ask the people who work there. Also, look at the third link I have above, it does have a few pictures as well as good descriptions on what they look like.