The book "The Lean Startup" defines what a startup is. No need for further explanations. Go and read the book if you have not. Save yourself time and money with it.
I personally prefer the definition that Steve Blank offers: "a startup is an organization formed to search for a repeatable and scalable business model". This to me makes it clear where you should focus. Once you have discovered this model, your startup turns into a "regular" company and your priorities turn to operations and scaling.