what is total internal reflection? and what does it occurs?

It occures when you have two different materials with differnt refractive indexes (n). The light moving from the higher n to the lower n (say from glass to air) if it at or below a certain angle will get refected back into the glass with no rays passing out into the air.

 

It can be seen to occure with a stream of water and a laser pen. But the most common example is fiber optic cables.

 

http://en.wikipedia.org/wiki/Total_Internal_Reflection is probably worth a read as well as http://en.wikipedia.org/wiki/Fiber_optics