LISP is the premier language for Artificial Intelligence applications. It is a dynamic language: editing changes take effect immediately, without the need for recompilation. It is primarily a functional language: all work can be done via function composition and recursion. There is no "main program:'' the programmer can call any function from the input prompt.
Someone once said that FORTRAN programs can be written in any language. When you learn LISP, you will grasp the essential nature of the language better if you first learn to write programs using only function composition and recursion. This approach is harder, of course, because it requires you to master a new way of thinking. This brief introduction enforces ``thinking in LISP'' by omitting the tools (loops and statement sequences) that would allow you to write FORTRAN-style programs.
LISP is an old language, and has been updated many times. The most recent standard, Common LISP, is a huge language. This introduction covers only the core of LISP, and should be accurate for virtually any LISP system.