The document discusses growth hacking, which refers to tactics and best practices for increasing user growth for startups. It provides the example of how Hotmail grew rapidly in the late 1990s by adding "PS: I love you. Get your free email at Hotmail" to the bottom of every email. This led to exponential growth. It then outlines frameworks for mapping the user lifecycle and measuring conversions at each step. It also discusses techniques like A/B testing, which Highrise used to significantly increase account signups. Finally, it lists growth hacking best practices such as using social proof and calls to action.