Garbage collection identifies the cells of memory that are in use; the remaining cells, which are not used for anything, are collected and added to the Free List. This automatic recycling of unused memory is a major advantage of Lisp; it makes it possible for programs to create (`` cons up'') new structures at will without having to worry about explicitly returning unused storage.
Identification of ``in use'' memory starts from symbols, which are always ``in use''. Symbols, in turn, may be bound to other data structures; these structures, and their sub-structures, must also be marked as being ``in use''.