Originally, the term Lisp referred to the Programming Language which was the brainchild of John Mc Carthy. The invention of Lisp dates back to 1958.
Now, when unspecified, the term Lisp either refers to Common Lisp (a language) or the Lisp family of languages, all of which share the Lambda Nature.
imgs.xkcd.com
"Lisp is the world's greatest programming language - or so its proponents think." - Guy Steele and Richard Gabriel, The Evolution Of Lisp, History of Programming Languages II, Addison-Wesley, 1996
Lisp pioneered a lot of things, making it one of the Ground Breaking Languages. Some well-known examples include Garbage Collection and the Null Object (NIL).
Well-known members of this family, alive and deceased, include:
Common Lisp (descended from Mac Lisp, Zeta Lisp, Franz Lisp and others)
XLisp
ISLisp
New dialects of Lisp:
Tiny Lisp (www.ugcs.caltech.edu 
)
(see also Modern Clean Lisp)
Lisp inspired languages include ...
The Association Of Lisp Users web site is a good starting point for Lisp information:
www.alu.org (Contents page)
www.lisp.org (History of Lisp)
Also see The Evolution Of Lisp.
I have written an introduction to Lisp that can be found at p-cos.net which you might find useful. -- Pascal Costanza
Obligatory Quotes:
A Lisp programmer knows the value of everything, but the cost of nothing. (Alan Perlis)
Anyone could learn Lisp in one day, except that if they already knew Fortran, it would take three days. (Marvin Minsky)
Lisp is a programmable programming language. (John Foderaro)
If you give someone Fortran, he has Fortran. If you give someone Lisp, he has any language he pleases. (Guy Steele)
Smug Lisp Weenies love to yell at each other about whether Scheme, which is, as far as the layman is concerned, a dialect of Lisp, is a Lisp or not. (see Is Scheme Lisp) The fact that none have done so here is a testament to the sanity of Wiki authors. Thank you all! -- Daniel Knapp
That'd be like doing your dirty laundry in public. For the plebs, we'll let it slide. :-)
Not to mention the fact that the list of things that 'smug lisp weenies love to yell about' is highly politicized and inaccurate for some reason, I think. I know a lot of lispers, and I don't know any who behave by the stereotypes presented here and elsewhere. I would say, as a language community, it is better behaved than most. Perhaps that is just me, though. It *is* skewed towards experienced and talented programmers (as are many other `niche' languages, for similar reasons), and this group as a whole has little time for some sorts of newbie behaviour. On the whole, this is understandable, and for the most part stays reasonable....
Lisp is like zen when you see it for the first time it's like something you already knew.
How is this sentence to be punctuated?
Lisp is like zen when you see it for the first time: It's like something you already knew. (my guess)
Lisp is like zen: When you see it for the first time, it's like something you already knew. (could be)
Lisp is like zen when you see it: For the first time, it's like something you already knew. (probably not)
Lisp is like zen, when you see it for the first time, it's like something you already knew. [not quite grammatically correct; needs a colon, semicolon, or fullstop after the first clause]
Lisp is, like, zen. When you see it for the first time, it's, like, something you already knew. (the valley-girl or stoner version)
Lisp is like zen, When. You see it for the first time. It's like something you already knew. (I'd really hate to be named When)
Lisp is. Like zen when you see it for the first time, it's like something you already knew. (c'mon, it's zen we're talking about. Zen is.)
Lisp is like zen. When you see it for the first time, it's like something. You already knew! (duh, it's that obvious)
Lisp is.
Ahh, does this mean that Lisp thinks? ;-)
No, no. 'Is' is Lisp.
Lisp is. Like zen when. You see it? For the first time it's like something you already knew.
An interesting thing about Lisp is that John Mc Carthy was a professor in a university who taught Lisp as a mathematical idea. Then one pupil (Steve Russell) commented that if only the function eval was implemented, then Lisp programs could be run. Maybe that's the reason Lisp is the simplest language...
This is a summary of a study of Lisp versus Java which compares software development time and resulting program characteristics for several tasks. It's modelled on a similar study comparing C++ and Java.
The "study" is comprised of self-selected Lisp developers in a single problem domain. The kindest thing I can say about it is that it's an anecdote, not a study.
See also: Ess Expressions Lost Ina Seaof Parentheses Cons Cell Forth Vs Lisp Is Scheme Lisp Implementing Lisp
See original on c2.com