How are POSIX Threads implemented on Linux?
Before Linux 2.6 they were essentially userspace threads, separate processes which were glued together, because the kernel had no real thread support. Edit: There was some limited support for kernel level threads (a clone() function) before 2.6, but it wasn't used with posix threads, only with an alternative thread library called linuxthreads.Since the arrival of NPTL (Native Posix Thread Library) the are kernel threads.
Threads created by pthread_create() on Linux have always been kernel-level threads. LinuxThreads didn't comply fully with POSIX (threads in a same process had different pid's, signal handling was different than what POSIX requires, ...), so NPTL was created to fix those issues.
There are libraries that implement user-level threads (e.g: GNU pth, the p is for Portable), but they don't use the POSIX thread API.