I didn't fully understand 37 Signals' advice to Have an Enemy until recently.

Sometimes the best way to know what your app should be is to know what it shouldn't be. Figure out your app's enemy and you'll shine a light on where you need to go.

When we decided to create project management software, we knew Microsoft Project was the gorilla in the room. Instead of fearing the gorilla, we used it as a motivator. We decided Basecamp would be something completely different, the anti-Project.

One bonus you get from having an enemy is a very clear marketing message. People are stoked by conflict. And they also understand a product by comparing it to others. With a chosen enemy, you're feeding people a story they want to hear. Not only will they understand your product better and faster, they'll take sides. And that's a sure-fire way to get attention and ignite passion.

I've explained Stack Overflow to hundreds of people, and by far the most effective way to explain what we do -- the way that causes people to visibly "get it" almost instantly, with a giant cartoon lightbulb practically appearing over their head -- is this:

We're like experts-exchange, but without all the evil.

I never appreciated how easy Experts-Exchange makes it for us. They are almost universally loathed. We don't just have a rival, we have a larger than life moustache-twirling, cape-wearing villain to contrast ourselves with.

No matter how much we may suck (and we try very, very hard not to suck), we can simply point to the experts-exchange website and we instantly become the hero. The good guy. The underdog.

I have absolutely nothing against Experts-Exchange. Realize that I've been a fan of the smackdown learning model for a long time; it's like kayfabe in professional wrestling. There are no hard feelings; this "rivalry" is mostly useful as a way to explain what it is we do. This internet is certainly big enough for the both of us -- big enough, in fact, for hundreds of Q&A websites.

That said, if you have an arch-enemy -- the more horrible and evil and larger-than-life the better -- consider yourself lucky. They're doing you a huge favor.

So, who's your arch-enemy?