More clarification for you on halves:
1964 and earlier proof coins had a mirror-like finish to them (no mint mark).
1965, 1966, and 1967 had "Special Mint Sets" (or SMS), no proof releases for those years.
1968 to the present have an "S" on the coin as the mint mark ("S" stands for San Fransisco, where they are minted.)
In 1968, 1969, and 1970, all of the coins with an "S" on them were considered proof-only releases. These 3 coins are also 40% silver.
The US Mint also made a special Bicentennial set in 1976 that featured 3 40% silver coins: A 40% Eisenhower dollar, a 40% silver quarter, and a 40% silver Kennedy half. These are silver, however there are also NON-silver proofs from this year (All of these have an "S" on them too).
The US Mint continued to make regular old clad proof coins with an "S" until 1991, then they started making a clad AND a silver proof set (each with an "S" on it) for each year starting in 1992.
Starting in 2002, the US Mint began minting coins specifically for collectors, which were never intended for circulation, they are called NIFC, or Not Intended For Circulation.