Extended Type Design.

[Walter Bright]

Bruno has answered your specific questions, so I'll take a more general 
tack.

A symbol is a name to which is 'bound' a value.

static int x = 3;

'3' is the value.
'int' is the type.
'x' is the symbol.
'static' is the storage class.

Here, we bind a new value to the symbol x:
     x = 4;

A storage class originally meant where the symbol is stored, such as in 
the data segment, on the stack, in a register, or in ROM. It's been 
generalized a bit since then. The main way to tell a storage class apart 
is that:
1) a storage class applies to the symbol
2) a type is independent of storage class, i.e. you cannot create a type 
that is "pointer to static" or "array of extern". Storage classes do not 
affect overloading, nor type deduction.

'invariant' and 'const' are type modifiers (aka type constructors), 
which mean when they are applied to a type, a new type is created that 
is a combination.

'invariant' is a guarantee that any data of that type will never change. 
'const' is a guarantee that any data of that type will never be modified 
through a reference to that type (though other, non-const references to 
that type can modify the data).