Premillennialism is simply the belief that we are living in the time prior to the Millennial Reign of Christ, which will begin at Jesus' Return, at which time He will begin to rule the nations with an iron scepter as outlined in Revelation 20 (verses 1-4.)
Post Millennialism is the view that Christ will not return until Christians have established the Kingdom of God on the earth. (See link here.)
Amillennialism is the belief that the thousand year time period is neither literal, nor is it future, but that Christ currently is reigning from David's throne in Heaven for a "long period of time" not necessarily one thousand years. (See link here.)
In this video, we explain why we are all premillennialists and why it matters for Biblical interpretation. Enjoy!