The term "proof" refers to a specimen striking of coinage for presentation, souvenir, exhibition or numismatic purposes. Pre-1968 proofs were struck at the Philadelphia Mint except for the rare occurance when a presentation piece was struck at a branch mint. Very few proof coins were made prior to 1856.
No proofs were minted from 1917 through 1935 except for a few 1921 and 1922 Peace Dollars. The Philadelphia Mint resumed striking proof coins in 1936 and continued until 1942. Striking was temporarily suspended from 1943 to 1949 and 1965 to 1967. Special Mint Sets were issued in 1965, 1966, and 1967. Minting of Proof Sets were resumed in 1968 at the San Francisco Mint and continues today.
Official Mint Sets are also specially packaged and sold to collectors. These sets contain specimens of each year's coins for every denomination used for each mint. Though uncirculated, no special attention is given to quality as is done for the proof coins. Official mint sets were produced starting in 1947 and continue today. No official Mint Sets were produced in 1950, 1982, and 1983.