tell me how

ok i borrowed a backup movie off a friend which worked in my mums dvd player which i havent got any of my movies to play in anyway before looking i put it in my drive to burn it and after it was finished i took it out and relised it was on a -r disk and my dvd writer is +r how did it work? also the copy i made played in my mums player?

 

It can read -R obviously, but it won't write -R.