PulseAudio is a sound server system for POSIX OSes, meaning that it is a proxy for your sound applications. It is an integral part of all relevant modern Linux distributions and is used in various mobile devices, by multiple vendors. It performs advanced operations on sound data as it passes between your application and hardware. Things like transferring audio to a different machine, changing the sample format or channel count, or mixing several sounds into one input/output, are easily achieved using PulseAudio. [1]
Like ALSA, PulseAudio is dinamically configured using PulseAudiomodules. pactl is the command line tool for loading and downloading modules. For example, to use the equalizer qpaeq that should be already installed in your computer if you are using PulseAudio, run:
pactl load-module module-equalizer-sink pulseaudio --kill && pulseaudio --start qpaeq &
Another important PulseAudio application is the pavcontrol that is a mixer/VU-meter that handles sound applications, input and output audio devices.