While baking, that is. I've always wondered why chocolate chips maintain their shape, even if they've been in the oven for a while. Well, I now know. It's a matter of fat. They contain less cocoa butter, or fat, and that means they will hold their shape at high temperatures. I don't quite know what the chemistry behind it is, but I'm guessing that fat melts at lower temperatures than say, cocoa powder, which would in turn force a piece of chocolate to lose it's shape.
At any rate, chocolate chips have less fat. Chocolate bars, on the other hand, don't. So it's important to bake with a chocolate bar instead of chocolate chips, if a recipe calls for it. If you want a silky smooth chocolate mixture, go for the bar. :)