Amillennialism is a specific position in regard to the Millennium, it says that the Millennium isn't intended to describe a literal period of time, but rather describes Christ's reign at the Father's right hand until the time of His coming.
Preterism is a position in regard to prophetic interpretation, sometimes contrasted with Futurism and Historicism. Historicism would argue, for example, that what St. John the Revelator wrote has had an ongoing fulfillment since his day to ours, the earliest Protestants were Historicists, which led Luther, Calvin (et al) to conclude that the Papacy fulfilled the eschatological role of Antichrist and Beast (not the person of the Pope, per se, but rather the office of Pope). Futurists would posit that all or most of everything in the Revelation will be fulfilled at a heretofore unspecified point in the future, Dispensationalists fall in this category (and they are also Premillennialists).
Preterism, on the other hand, states that much (partial) or all (full) of what St. John wrote, or what Jesus spoke about in the Olivet Discourse (etc) had its fulfillment, or was primarily about, stuff that took place in the first century and the time of John himself. Hyper-Preterism goes so far as to argue that the Parousia--the Second Coming--occurred in 70 AD when the Roman army destroyed the Temple, they believe this was a visitation of Judgment, the return or appearing of Christ, against the old order represented by Jerusalem and the Temple. Hyper-Preterism is regarded as heretical by mainstream Christianity as the Historic Creeds are clear that the Lord will return at the end to raise the dead bodily (Hyper-Preterism denies the resurrection of the body, which is, again, heretical).
The two involve different aspects of eschatology, one could be both Amillennial and a Preterist, one could be Amillennial but not a Preterist. Luther, Calvin and other early Protestants were Amillennialists and Historicists, one can also be Amillennial and a Futurist.
-CryptoLutheran
