There's a lot of fuss over this "growth hacking" thing lately.
Ideally, it describes a measurement-driven process which is sensitive to the fact that apparently small issues can have a large impact on how users react to your marketing. It tries to be more scientific and accountable, to treat the overall marketing engine as something to be tuned and optimized, and to focus on reaching a target customer instead of placing big bets on monolithic projects that are hard to measure.
Customer development is, broadly speaking, the process of doing directed work to improve your understanding of your customer. We want to know about what problems they have, how they see and interact with those problems, and what their values are.
You can make some good guesses based on experience, but people will always surprise you. There's often a huge disconnect between how businesses conceptualize their products and their sales process, and what is actually happening from the customer's perspective.