C/C++ Storage Classes

auto	Default storage class for all local variables. Can only be used within functions, i.e., local variables.
register	Used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and can't have the unary '&' operator applied to it (as it does not have a memory location). It should only be used for variables that require quick access such as counters. It should also be noted that defining 'register' does not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register depending on hardware and implementation restrictions.
static	Instructs the compiler to keep a local variable in existence during the life-time of the program instead of creating and destroying it each time it comes into and goes out of scope. Therefore, making local variables static allows them to maintain their values between function calls. The static modifier may also be applied to global variables. When this is done, it causes that variable's scope to be restricted to the file in which it is declared. In C++, when static is used on a class data member, it causes only one copy of that member to be shared by all objects of its class.
extern	Defines a global variable that is visible to ALL object modules. When you use 'extern' the variable cannot be initialized as all it does is point the variable name at a storage location that has been previously defined.
mutable (C++)	Applies only to C++ class objects. It allows a member of an object to override constness. That is, a mutable member can be modified by a const member function.