pthreads is a C library for POSIX that has been ported to other systems.
Because pthreads is written to be portable it misses some fancy features,
but is much more feature rich than threading classes you get with languages.
Using those more feature rich threading libraries allows for more clean
and effective multithreading. For example if you have want to parallellise
a loop you can in Java use JNI to all C methods to uses pthreads and the
barrier synchronisation method to to wait for all threads to join.