One thing I really hate is "co-founder." I like "founders" better. Because before you know it, there's a "founder" and then there's "co-founders", and its all kind of bullshit over nothing (except money, and the sudden appearance of unwritten liquidation preferences, and the long slow road to self-destruction that those things breed). I don't really have a problem with equity being split asymmetrically, but I think everything else should be fair and proportional.
I think it's the ambiguity and uncertainty that bugs people.
I think of founders as the people who decide to do the startup. If they decide and then bring people on before they have gotten very far, I would consider those cofounders. There's a little bit of seniority difference but each person would be just as critical.
For instance, I've got a one-man startup GeekStack (http://geekstack.com). I'm the founder. I don't have much done, just organizing, planning, and a marketing website. I'm talking to some of my college friends about joining me, and at this point, they would be cofounders. If they outwork me or turn the project in a direction I hadn't thought of, I'd probably consider them equal founders. If they join me in 4 months when I have printed my first set of cards, they would be early employees. Does that help?
BTW, anyone with graphic design skills interested in making trading cards of CS heroes, my email's in my profile :)
The difference is that founder is singular, where co-founder is plural. I am a co-founder of a company because I co-founded it with another person. In my mind it has nothing to do with who's done what.
