The Millennial Reign is discussed in Revelation chapters 19-20. It starts with a great feast known as "The Marriage Supper of the Lamb" where Jesus Christ is married to His Bride (the Church). Shortly thereafter (at Armageddon) he slays the world's armies, captures The Antichrist and The False Prophet and casts them into the Lake of Fire (Hell), and binds Satan in a bottomless pit for 1,000 years (thus the name "millennial reign"). Only those who went through the preceding Tribulation period and did not take the Mark of the Beast remain alive.
After the 1,000 years ends, Satan is loosed for "a little season" (the duration of which is not specified in the text), whereupon he seduces the nations to fight against God one final time, but is defeated and cast into the Lake of Fire, along with Death and Hades. All who have ever lived are then judged by checking to see if their names are in the Book of Life: if it is they enter Heaven, if not they are cast into the Lake of Fire.