This is a difficult one because ska has come to mean so many different things 
to different people.  You could call Operation Ivy or the Toasters ska as well 
as The Specials, Madness, and the English Beat, however the real deal is the 
old sixties Jamaican sound.  Here is somewhat of a history as I understand it.
The Skatalites claim to have invented the ska sound which was the first to use 
the offbeat guitar we have come to recognize in reggae.  The ska sound is a bit 
faster and jazzier than the reggae sound and is much more dependent on the 
American R&B style of the fifties and early sixties.  In the later fifties 
Jamaican djs began travelling to New Orleans to buy American R&B to play at 
their sound system dances; some of these djs included the famous Coxsone, 
Prince Buster, Duke Reid etc.  The trick was to find a record that no one had 
or was able to find and to play it to attract people away from the other sound 
systems.  Soon these djs began to record their own tracks to insure that they 
would be the only ones with a particular record.  They would record the track 
and then produce a single copy of it at which point the dj would play the 
record to death until a demand arose for it.  Then the record was released 
commercially to maximize sales.
As far as the sound of ska itself, it can best be described as an amalgamation 
of a Caribean beat with American R&B underpinnings.  Anyone who has ever seen 
the Skatalites live realizes that the ska sound is very similar to jazz in that 
it is a single steady rhythm with lots of instrumental explorations going on.  
After a number of instrumental releases the djs began producing vocal tracks by 
artists that would later become reggae greats such as Bob Marley, Desmond 
Dekkar, the Maytals, Jimmy Cliff etc.  The vocal ska records are much more 
blatantly American influenced with lots of "do-wops" in the backround and "my 
baby left me" sort of subjects.  
After a few years from the influence of American soul the ska sound began to 
slow down and focus more on individual singers; this sound was called 
rocksteady.  The rocksteady songs are more socially conscious and the 
influences of rastafarianism began to creep in.  Eventually, the sound slowed 
even further and became reggae by the early seventies.
Some records to check out:
The Wailers--There was a really cheap but poor quality comp out a bit ago 
called "Early Music" which had all the most important tracks of the ska days of 
the Wailers.  However, a better buy is the Heartbeat label two cd release with 
all of the early Wailers tracks.
Many Comps--"Club Ska 67," "Intensified" vols. 1 & 2, "Dancecrasher: Ska to 
Rocksteady," and many compilations on the Heartbeat and Trojan labels.  Also, a 
couple of years ago Heartbeat released an excellent comp of Duke Reid's ska and 
rocksteady stuff.
I know this is brief but I hope it helps.
Keith