What is the difference between Linux and RTOS ?

Linux and RTOS are two distinct concepts.

Linux is the name given to a specific operating system.

An RTOS (Real-Time Operating System) is a specially designed TYPE of Operating System. Where a 'normal' Operating Systems schedule resources/processing time to programs when they are available to the system, whereas an RTOS specializes in allowing programs to run in 'real time', meaning they are able to respond to changes (typically from an external source) as close to instantly as possible.

Typically, you would find an RTOS in situations where the computer has to react instantly to changes, such as an industrial control system or systems that monitor and control the power grid. If the operating system prevented the program from responding in a timely fashion, the program would be worthless.

Real-time Operating Systems are typically very small, optimized systems, whereas a standard OS is very large, feature-laden and has thousands of different features that run at once.

There IS a pared-down version of Linux known as "RTLinux" which falls into the category of an RTOS, but it's not the version of Linux most people are used to seeing. It's very purpose-build, optimized and fast, unusable by anyone but system designers who can work with such an OS.