The volatile keyword indicates that a value may change, even if it does not appear to be modified.
For C#/Java, "volatile" tells the compiler that the value of a variable must never be cached as its value may change outside of the scope of the program itself. The compiler will then avoid any optimisations that may result in problems if the variable changes "outside of its control".
In C/C++, "volatile" is needed when developing embedded systems or device drivers, where you need to read or write a memory-mapped hardware device. The contents of a particular device register could change at any time, so you need the "volatile" keyword to ensure that such accesses aren't optimised away by the compiler.