Real-time systems are special-purpose operating systems.
A real-time system is used when there are rigid time requirements on the operation or a processor or the flow of data, therefor it's often used as a control device in a dedicated application.
A hard realtime system GUARANTEES that critical tasks completed on time.
Modern operating system features like VM is (almost)never used in a HRTOS, also, the data is usually stored in a ROM.
A "good" use of HRTOS is found in, for example, weapon systems.
As you might have understood the HRTOS is tightly linked with the H/W.
In soft real-time systems there are still time-critical tasks. They get priority over other tasks until the critical task is completed.
Solaris is a SRTOS, and is by far superior to Linux on good H/W(PCs sucks rock).
This is VERY short description, if you want to know more you'll have to wait for me and my brother to set up our homepage, not a time-critical task I hope