Traditional civil Secondary Surveillance Radars interrogate in Mode A and Mode C.
A Mode C capable radar transponder will encode the altitude in the reply to a radar Mode C interrogation; there resolution of the Mode C altitude is 100ft.
The Mode A reply contains the squawk code.
There are several problems with Mode A/C technology.
One of the problems is that all aircraft reply when they are interrogated. When several aircraft fly close to each other (for example in a holding stack), all their replies overlap, making it difficult for the radar to distinguish the replies. This is called garbling.
To address this and other problems, Mode S was introduced. 'S' standing for Selective.
Mode S radars can issue addressed interrogations; they can specifically ask a single transponder to reply. They do this by encoding the unique 24 bit address of the aircraft in the interrogation. The addressed aircraft will then reply to the question that was asked by the radar. In Elementary Mode S there are 4 basic questions that a radar can ask:
who is there? reply with unique 24 bit address
what altitude are you at? reply in 25ft resolution
what is your squawk code?
what is your identity? reply aircraft ID / call sign
In Enhanced Mode S adds a whole list of additional questions that can be asked, including airspeed, groundspeed, magnetic heading, track angle rate, selected altitude etc.
Mode S radars are a lot more sophisticated that the traditional Mode A/C radars and they also require more sophisticated transponders.
In order to be able to ask the right question at the right time to the right aircraft, the radar needs to keep track of who is flying where within their coverage area. But they are more accurate and allow for more detailed information being extracted from the aircraft.