Audio in GNU/Linux has come a long way and nowadays functions fairly well when it comes to the simplicity that users migrating from Windows are accustomed to; but there are still some nuances and terms that new users may not be familiar with.
ALSA is the framework that sound drivers communicate through. ALSA is nowadays the basis for all sound in a GNU/Linux system. Typically ALSA can only send sound to one place at a time
PulseAudio is included with practically every major pre-built GNU/Linux operating system. PulseAudio acts as network-capable a sound server that directs the sound it receives from ALSA, to your speakers, headphones, etc. It supports many sound devices and per application volume control.
JACK is another Sound Server similar to PulseAudio, but is more commonly used among DJ’s and audio professionals.