What is my Mac username?

My username in the Mac login screen ("My Name", there is a space) doesn't match the one in Finder and terminal ("anothername"). Which one is my real username? The reason I'm asking is that I want to share media files over my home network, and when I was trying to access the media from another computer, it ask me for username and password. I don't know which one is my username I only know the password. I tried both and neither worked. Always says wrong username or password. I'm sure I know my password because that's how I login to my Mac. Could there be a third username? I'm completely lost.