RtLinux is an attempt to get the LinuxOperatingSystem kernel to respond faster. Linux is very much not a RealTimeOperatingSystem, and contrary to what the people waving instrumented kernels will tell you, it is very difficult to turn it into one ("low latency" patches tend to give response times in the milliseconds). RtLinux gets around this by implementing its own real time operating system which runs a standard Linux kernel as the idle task. This means that you still get to use all the Linux kernel features, you still get very fast response times, and the only cost of all this is that real time tasks are things very much unlike normal Linux tasks. This sounds needlessly complicated, but it works remarkably well.
[NOTE: RTLinux has been acquired by -- who else? -- Wind River Systems. There is still a GPL version of this OS RtLinuxFree (www.rtlinuxfree.com), but for any commercial applications you are gonna end up with Wind River Linux. Suck it up, yo.]
RTAI (RealTimeApplicationInterface?) is closely related and was forked from an early version of RtLinux. Anyone considering using a real-time Linux extension should check it out too. Many choose rtai for reasons pertaining to abuse of the GPL and of the patent system.