Pthreads (POSIX Threads) is a standardised C-based API for creating and manipulating threads. It is currently defined by POSIX.1-2008 (IEEE Std 1003.1, 2013 Edition / The Open Group Base Specifications Issue 7).
The API is mostly covered by the header documented at http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/pthread.h.html and the behaviour by http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09
See https://en.wikipedia.org/wiki/POSIX_Threads for more details and further reading. The POSIX.1-2008 Base Definitions is available online at http://pubs.opengroup.org/onlinepubs/9699919799/
POSIX Threads is also covered extensively in Programming with POSIX Threads by David Butenhof.
A port to MS-Windows (x86/x64) is available at: https://sourceware.org/pthreads-win32/
pthreads is also the name of an Object Oriented API that allows user-land multi-threading in PHP created by Joe Watkins