This is Amarok, KDE's music player, which comes installed with Kubuntu. Amarok can be installed to other Linux distros from the software repositories, and is also available for Windows.
I had been using VLC media player for many years, and still use it for videos, but Amarok is the first music player I've come across that has caused me to switch from VLC for playing music.
It couldn't be easier to operate. All I do is right click on whatever I want to play from my mp3 directory (that's "folder" in Windows-speak) which you see in the left-hand column of the screen, and select "replace playlist." Amarok then starts automatically playing whatever I've selected, and I line up more music by right clicking on other directories in the left-hand column and selecting "add to playlist."
It uses up a fair amount of system resources when the window is open, but it minimises to the system tray when I click the "close" icon, and then uses very little system resources.
Edited by Al1000, 18 April 2015 - 01:06 PM.