Business Development, or "biz dev" requires an understanding your product, your market, your competitors, your suppliers, your customers and everything else you learned from doing your market analysis. Once you get an understanding of where your market is heading, you can figure out what types of activities will enhance your company, hurt your competitors, grow your customer base, cut costs, and all the other things that companies try to do to maximize profits. These activities might include joint development projects (even with competitors), investments in promising technologies, mergers and acquisitions (M&A), and similar sorts of strategic business moves.
While it is important to understand what activities will benefit your company, the real key to business development is networking. For doing biz dev I have talked to venture capitalists, presidents of 5-person start-ups, VPs of major public corporations, analysts, you name it. Sometimes the job consists of coming up with a business concept and knowing who to call (in order to talk to the VP of one company I called someone I once worked with who once worked at that company, who connected me to another person, who got me the secretary's name of the man I needed to talk to). Other times, people call you and you have to be prepared to act (an analyst once called me once asking if I would be interested in buying one of my competitors' divisions. "Yes" is always a good response to this kind of question, even if you're not really interested, but the important thing is that the analyst knew to call me with the opportunity).
So networking is the key to business development. And as everyone knows, networking is the key to finding a new job. Therefore, Business Development is one of the best ways to land a new and better job!
It's true. Take a look at this post here where I called up the exec VP of a company for a joint business opportunity and the call ended with him asking for my resume. Once you get up to middle management, advancement to the next rung just isn't listed on Monster.com.
So, the bottom line: if at all possible work your way into Business Development to get your name out there and network your way up in the corporate ladder. It's one of the best ways to help yourself while you help your company.
Ex-colleague Rorschach worked in this area and could also add a few comments (hint, hint).
Update: Rorschach has left some great business tips and insight in the comment section, so go read it (maybe I can talk him into being a regular contributor?). I will be expanding on some of his points tomorrow with a section on sales.