What are the differences between Clojure, Scheme/Racket and Common Lisp

Matt's explanation is perfectly fine -- and he takes a shot at a comparison to C and Java, which I won't do -- but for some reason I really enjoy discussing this very topic once in a while, so -- here's my shot at an answer.

On points (3) and (4):

Points (3) and (4) on your list seem the most interesting and still relevant now.

To understand them, it is useful to have a clear picture of what happens with Lisp code -- in the form of a stream of characters typed in by the programmer -- on its way to being executed. Let's use a concrete example:

;; a library import for completeness,
;; we won't concern ourselves with it
(require '[clojure.contrib.string :as str])

;; this is the interesting bit:
(println (str/replace-re #"\d+" "FOO" "a123b4c56"))

This snippet of Clojure code prints out aFOObFOOcFOO. Note that Clojure arguably does not fully satisfy the fourth point on your list, since read-time is not really open to user code; I will discuss what it would mean for this to be otherwise, though.

So, suppose we've got this code in a file somewhere and we ask Clojure to execute it. Also, let's assume (for the sake of simplicity) that we've made it past the library import. The interesting bit starts at (println and ends at the ) far to the right. This is lexed / parsed as one would expect, but already an important point arises: the result is not some special compiler-specific AST representation -- it's just a regular Clojure / Lisp data structure, namely a nested list containing a bunch of symbols, strings and -- in this case -- a single compiled regex pattern object corresponding to the #"\d+" literal (more on this below). Some Lisps add their own little twists to this process, but Paul Graham was mostly referring to Common Lisp. On the points relevant to your question, Clojure is similar to CL.

The whole language at compile time:

After this point, all the compiler deals with (this would also be true for a Lisp interpreter; Clojure code happens always to be compiled) is Lisp data structures which Lisp programmers are used to manipulating. At this point a wonderful possibility becomes apparent: why not allow Lisp programmers to write Lisp functions which manipulate Lisp data representing Lisp programmes and output transformed data representing transformed programmes, to be used in place of the originals? In other words -- why not allow Lisp programmers to register their functions as compiler plugins of sorts, called macros in Lisp? And indeed any decent Lisp system has this capacity.

So, macros are regular Lisp functions operating on the programme's representation at compile time, before the final compilation phase when actual object code is emitted. Since there are no limits on the kinds of code macros are allowed to run (in particular, the code which they run is often itself written with liberal use of the macro facility), one can say that "the whole language is available at compile time".

The whole language at read time:

Let's go back to that #"\d+" regex literal. As mentioned above, this gets transformed to an actual compiled pattern object at read time, before the compiler hears the first mention of new code being prepared for compilation. How does this happen?

Well, the way Clojure is currently implemented, the picture is somewhat different than what Paul Graham had in mind, although anything is possible with a clever hack. In Common Lisp, the story would be slightly cleaner conceptually. The basics are however similar: the Lisp Reader is a state machine which, in addition to performing state transitions and eventually declaring whether it has reached an "accepting state", spits out Lisp data structures the characters represent. Thus the characters 123 become the number 123 etc. The important point comes now: this state machine can be modified by user code. (As noted earlier, that's entirely true in CL's case; for Clojure, a hack (discouraged & not used in practice) is required. But I digress, it's PG's article I'm supposed to be elaborating on, so...)

So, if you're a Common Lisp programmer and you happen to like the idea of Clojure-style vector literals, you can just plug into the reader a function to react appropriately to some character sequence -- [ or #[ possibly -- and treat it as the start of a vector literal ending at the matching ]. Such a function is called a reader macro and just like a regular macro, it can execute any sort of Lisp code, including code which has itself been written with funky notation enabled by previously registered reader macros. So there's the whole language at read time for you.

Wrapping it up:

Actually, what has been demonstrated thus far is that one can run regular Lisp functions at read time or compile time; the one step one needs to take from here to understanding how reading and compiling are themselves possible at read, compile or run time is to realise that reading and compiling are themselves performed by Lisp functions. You can just call read or eval at any time to read in Lisp data from character streams or compile & execute Lisp code, respectively. That's the whole language right there, all the time.

Note how the fact that Lisp satisfies point (3) from your list is essential to the way in which it manages to satisfy point (4) -- the particular flavour of macros provided by Lisp heavily relies on code being represented by regular Lisp data, which is something enabled by (3). Incidentally, only the "tree-ish" aspect of the code is really crucial here -- you could conceivably have a Lisp written using XML.

Atoms are literals, constants with their own name for value. What you see is what you get and don't expect more. The atom cat means "cat" and that's it. You can't play with it, you can't change it, you can't smash it to pieces; it's cat. Deal with it.

I compared atoms to constants having their name as their values. You may have worked with code that used constants before: as an example, let's say I have values for eye colors: BLUE -> 1, BROWN -> 2, GREEN -> 3, OTHER -> 4. You need to match the name of the constant to some underlying value. Atoms let you forget about the underlying values: my eye colors can simply be 'blue', 'brown', 'green' and 'other'. These colors can be used anywhere in any piece of code: the underlying values will never clash and it is impossible for such a constant to be undefined!

^{taken from http://learnyousomeerlang.com/starting-out-for-real#atoms}

With this being said, atoms end up being a better semantic fit to describing data in your code in places other languages would be forced to use either strings, enums or defines. They're safer and friendlier to use for similar intended results.