Not a dumb question at all...I did a bunch of spin classes over the summer with a friend (I think she might have been hitting on me....but I digress) and we did lots of standing on the bike. Since then my climbing has gotten a lot better (hard to do in FL). Anyway, a spin bike has a more upright position than a road bike or tri bike and will use different muscles than you would use by riding your normal bike. A computrainer keeps you in that more forward position, brining your hip flexors, etc. more into the equation to more closely simulate actual road miles. There's nothing wrong with spin classes, though, since they're still a lot better than eating turkey and getting fat!

Training with a computrainer is very different from a spin class. The spin class is a good general workout from an aerobic perspective. A computrainer on the other hand can be used to track cycling specific information, including power that can be used, along with custom loaded courses to improve certain aspects of your cycling. The club I've trained with does spin classes using a group of computrainers, with the workouts varied to whatever everyone is working on, whether that be strength, endurance, etc.