what is the difference between rtlinux and linux
Desktop linux - a linux distribution installed on your desktop machine and intended for day-to day use, like running an application at work, browsing, reading e-mail, writing documents, watching movies etc
Embedded linux - a linux distribution for embedded devices that control for example home appliances such as DVDs, microwave ovens, washing machines - this linux has a very small footprint, since it has to run inside a microcontroller with low resources (such as ARM, typically speeds of MHz or tens of MHz, memory 4-8-16 MB) an it is reduced to the minimum components needed to do the job.
RTLinux - Real Time linux - as above, but with real-time constraints - the designer guarantees maximum response times for any operation. RT linux is used to control industrial machinery, in automotive applications etc., where you must guarantee time responses and stability in order not to break expensive machinery and potentially endanger human lives.