POSIX Threads

Specification of The Open Group:
http://www.opengroup.org/onlinepubs/007908799/xsh/threads.html

Supports

creation of threads and waiting for their completion,

mutual exclusion (necessary to access shared data structures),

condition variables (useful to signal that a condition is satisfied another thread is waiting for),

read/write locks to synchronize concurrent read and write accesses to a shared data structure.

Free Implementations:

GNU Portable Threads:
http://www.gnu.org/software/pth/

Next Generation POSIX Threading by IBM:
http://oss.software.ibm.com/pthreads/