Any coin minted before 1965 is made of 90% silver - 1964 Kennedy halves are the only year they were minted in 90% silver. In 1965, Kennedys alone were minted in 40% silver (for the collector market - half dollars were not widely circulated by then), and in the following year to the present, they are nickel-clad copper, as is all circulating currency.
The difference is largely circulation; other halves were circulated more, and the relief on the obverse and reverse are worn down to a greater degree (worn down coins are called "slicks" by coin collectors and dealers, "soft coins" by magicians).