IS and NIS Cable Difference

In its most simplest form

IS (Intrinsically Safe circuit) and NIS (non Intrinsically Safe circuit) cables are designated, colour coded and run differently to standard or non intrinsically Safe cables to ensure separation of the two types and protection of the intrinsically Safe circuit.

The intrinsically safe barrier or galvanic isolator is there to limit the available power so an accidental short circuit on an intrinsically safe circuit in a gas or dust atmosphere cannot result in an explosion due to a spark or heat.

http://en.wikipedia.org/wiki/Intrinsic_safety

Plenty of other links and guides are available on the internet or on hazardous area equipment supplier websites.

...........and do make sure the correct level of training is received before designing, installing or modifying electrical installations in hazardous areas.