Rtai is a realtime extension with a broad variety of services which make realtime programmers' lifes easier. some of them are

choose between fifo and rr scheduling policies on a per task basis.
extended posix apis and message queues, in kernel and user space
semaphores can be typed as: counting, binary and resource
a mailbox implementation
lxrt for hard real time in user space
this package contains the development files (header files, rtai-config and static library).