Google Analytics identifies each visitor to a website with a unique ID, and the first time someone reaches a site from a new device or browser, that visit counts as a New User. Every new ID Google Analytics registers counts toward this metric. Since the ID lives in the browser, and by extension the device, the same person visiting from a different device or browser shows up as another new user with another new ID. Visit again from that same browser, though, and Google Analytics counts it as a returning user instead.
