OSS and ALSA
OSS or Open Sound System is a framework for audio on GNU/Linux and *BSD systems. The GNU/Linux community eschewed OSS after improvements to the code were made proprietary, though these were eventually re-released open source.
OSSv4 is still used on FreeBSD and miscellaneous GNU/Linux distros.
ALSA or Advanced Linux Sound Architecture is the more powerful replacement framework. It is not itself a sound server like PulseAudio, it is just the Linux kernel code for handling sound as an API. PulseAudio and JACK are built on ALSA.