The answer of Peter is correct, but a detail is missing. I checked the system and it seems also that if a user sign in at one point, and sign in again the same day or another day, it will not show one more visitor. For example, I went on the website as user at 11.45pm and also at 00.20am, so 35 minutes later (but the day after), and the number of Visitors was the same.
So, from my understanding, the number of visitors is not only the number of users who signed in but - in reality - the number of users who sign in for the first time. In conclusion, it appears that the number of Visitors is the number of users.