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.)
"And I saw an angel coming down out of heaven, having the key to the Abyss and holding in his hand a great chain. 2He seized the dragon, that ancient serpent, who is the devil, or Satan, and bound him for a thousand years. 3He threw him into the Abyss, and locked and sealed it over him, to keep him from deceiving the nations anymore until the thousand years were ended. After that, he must be set free for a short time.4I saw thrones on which were seated those who had been given authority to judge. And I saw the souls of those who had been beheaded because of their testimony about Jesus and because of the word of God. They a had not worshiped the beast or its image and had not received its mark on their foreheads or their hands. They came to life and reigned with Christ a thousand years. 5(The rest of the dead did not come to life until the thousand years were ended.) This is the first resurrection. 6Blessed and holy are those who share in the first resurrection. The second death has no power over them, but they will be priests of God and of Christ and will reign with him for a thousand years."
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!