What is growth hacking all about? Similar to marketing or completely different?

Growth hacking is a term for building your customer count using short-term, quick feedback programs. It's generally deployed during a product's growth phase and often artificially increases the number of users without necessarily increasing paid users.

Wikipedia states:
Growth hacking is a process of rapid experimentation across a range of marketing channels and product changes to identify the most effective ways to grow a business. The term is typically applied to customer acquisition, retention and monetization strategies used by newly-formed companies, which don’t have the resources to employ standard corporate marketing strategies. Growth hackers are marketers, engineers and product managers that are trained to specifically focus on building and engaging the user base of a business.[1] Growth hackers often focus on low-cost alternatives to traditional marketing, e.g. using social media and viral marketing instead of buying advertising through more traditional media such as radio, newspaper, and television.[2]
Source: https://en.wikipedia.org/wiki/Growth_hacking

Growth hacking broken down and explained: http://www.datasciencecentral.com/profiles/blogs/data-scientist-shares-his-growth-hacking-secrets .

Read: https://growth-hacking.zeef.com/david.arnoux