Basically a typedef has exactly the same syntax as an object declaration except that it is prefixed with typedef. Doing that changes the meaning of the declaration so that the new identifier declares an alias for the type that the object that would have been declared, had it been a normal declaration, would have had.
A typedef is scoped exactly as the object declaration would have been, so it can be file scoped or local to a block or (in C++) to a namespace or class.
e.g.
Declares an int:
int a;
Declares a type that is an alias for int:
typedef int a_type;
Declares a pointer to a char:
char *p;
Declares an alias for a char *:
typedef char *pChar;
Declares a function pointer:
int (*pFn)(int);
Declares an alias for the type that is ‘pointer to a function taking int and returning int‘:
typedef int (*pFunc)(int);