The latter have special purposes. An atom may contain digits but must not begin with one. Thus, c23 is a fine atom, but 23c is not. Any expression enclosed in single quotes is also an atom. A variable is anything beginning with a capital or an underscore.
Structures come in various flavours.
A compound term consists of a functor and one or more terms called arguments . Sometimes a functor is allowed to appear between its arguments. This is only allowed if the functor has been declared to be a so-called operator . Some common functors have already been declared as operators. The tail is always itself a list.