Historically, many independent thread implementations by different hardware vendors were present. This caused portability between different Hardware problematic and a common interface was needed to be standardized.
The IEEE POSIX 1003.1c standard was a step in this direction. Implementations that follow the above standard are termed as POSIX threads (or Pthreads in short).
The below Link provides information on Posix threads and is an excellent source of information for Posix threads
Pingback: Advantages and Disavantages of Threads | Hitch Hiker's Guide to Learning