About packages and namespaces:

Package is essentially a collection that maps symbols to values. In CL packages have (at least) two namespaces: one for functions, other for any kind of variables. The reason for this is that when you write the form

    (fun #'a b)

you and the compiler can be sure that 'fun' and 'a' are both meant to be functions. There are both advantages and disadvantages to this.