The top business undergrads generally do not actually major in business; they go to top-tier undergrads and major in whatever they want. (The major exception is Wharton.) From there, most of them go into one of two fields.
Investment banking is, essentially, investing other people's money for them, making money for them, and taking a cut of it. This is where the big money is, but it's also extremely time consuming.
Consulting is basically lending an outside opinion to situations. You will sign on with a consulting firm like McKinsey or Bain. They'll take care of your training -- in theory, anyway -- and then assign you out to other companies. For example, you might be assigned to a project with United, where you need to assess their costs and expenses and find ways to cut those down.
Extremely selective jobs -- so selective that I don't know anybody who's gone straight into it -- are in so-called "private equity." There are two kinds of PE. Leveraged buyouts essentially involve buying out a failing company at a cheap price (it is, after all, failing), turning it around, and selling it at a higher price. Venture capital involves finding entrepreneurs with good ideas, helping fund and guide their business, and then selling your interest at a higher price. For example, a young group of promising scientists form a group; you assess their talent and decide that with a little business guidance and (mostly) startup money, they're going to be the next great biotech company. You give them the money, and they give you 50% of their company. If you're right -- or, at the very least, if you can trick the next buyer into thinking you're right -- then that company is going to skyrocket in value and you can then sell your 50% for much, much more than the initial money you invested.
All of what I've described are extremely theoretical ideals. Others are in a better position to address the actualities of the experience.