What is xem nem?

Could you please elaborate on what exactly is XEM Nem? I've heard it mentioned in the cryptocurrency sphere, but I'm not entirely clear on its purpose, functionality, or how it differs from other digital currencies. Could you provide a concise yet informative explanation of XEM Nem, including its key features, potential use cases, and any notable achievements or milestones it has reached so far? Additionally, how does it compare to other popular cryptocurrencies in terms of speed, scalability, and security?