A “generalized variable” or “place form” is one of the many places in Lisp memory where values can be stored. The simplest place form is a regular Lisp variable. But the cars and cdrs of lists, elements of arrays, properties of symbols, and many other locations are also places where Lisp values are stored.
The setf form is like setq, except
that it accepts arbitrary place forms on the left side rather
than just symbols. For example, (setf (car a) b)
sets the car of a to b, doing the same
operation as (setcar a b) but without having to
remember two separate functions for setting and accessing every
type of place.
Generalized variables are analogous to “lvalues”
in the C language, where ‘x =
a[i]’ gets an element from an array and
‘a[i] = x’
stores an element using the same notation. Just as certain forms
like a[i] can be lvalues in C, there is a set of
forms that can be generalized variables in Lisp.