Bottom line, it's based on the radical idea that you can find out more about "the market" for your "product" by getting out and talking to customers than from complex market research. And it gives hints and structure for these conversations.
It focuses on the "pain" the product meets (the use) - And on narrowly targeting the "earlyvangelists" (people dying to use your product) for these conversations.
- Lean Startup
- I love how clearly Cindy Alvarez writes http://www.cindyalvarez.com/best-practices/customer-development-interviews-how-to-finding-people
Unrelated, but since this post is on startups - On 5 slide venture pitch