module
when you declare in a module with 'dim' the variable is only seen by the module. When you declare in a standard code module with 'public' the variable becomes 'global' (i.e. able to be seen by any other code module or form module).
level
when you declare a variable, you can do so in the general section. A variable declared in this section can be seen by any sub in that module. A variable declared within a sub can only be seen by that sub.