In theory these are called storage classes. These will define the scope and life time of variables or functions.

There are mainly 4 of them:

auto is the default storage class for local variables.

register is 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 say a word.

static can also be defined within a function or as a global variable. If this is done, the variable is initalised at compilation time and retains its value between calls ie, inside the function. Because it is initialsed at compilation time, the initalistation value must be a constant. This is serious stuff - tread with care.

extern defines a variable as global one ie, that is visable to all files irrespective of where it is defined. When you use 'extern' the variable cannot be initalized as all it does is point the variable name at a storage location that has been previously defined.

A static variable in a function is not instantiated at compile time. Rather, it is done when the first call to that function is made. Thereafter, its value can change but it will persist between calls to that function.

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