The above chart summaries a form of agile software development called Scrum. There are others flavors of agile, such as XP. Regardless of the process, the overriding principals remain the same. Those principals are listed here agilemanifesto.org and discussed in the following OnStartups.com article.
Apr 14, 2006
Agile Software Startups: The Myth Of The Perfect Business Plan
By Dharmesh Shah
Lets say I had this idea for a great software product. If I told you I was going to spend the next 3-4 months writing (and rewriting) the product specification and design documentation and do everything I could to address what I thought would be the key issues, and then expected to actually use that document to build a near-perfect product that was going to succeed in the market, you’d likely dismiss me as being naïve or inexperienced or both.
Why? Because, clearly, if we’ve learned anything about software development it’s that the process is difficult to completely plan for and much of what actually happens has to be emergent and “iterative.”