Again, Wikipedia can be your friend.
Mode C is an enhancement to mode A: it has all the info from mode A (which is 4 octal digits in 12 bits plus an 'ident') and on top it also carries altitude info from the encoder.
Mode S is quite recent, can carry lots more info (like gps coordinates, registration, flight number and much more). Amateurs capture the returns from mode S transponders to create websites like flightradar24.com
As a microlighter I am little acquainted with general aviation, but to my understanding the average 172-category plane in Western Europe carries a mode C transponder. Mode S being rather recent, its equipment is rather expensive.