The Millennium is when Christ returns and rules the Earth for 1000 years. Pre-millennialists believe Christ will return before the 1000 years.
Post-millennialists believe Christ will return after the 1000 years.
Amillennialists don't believe in it at all.
and Pan-millennialists believe that no one can know the future, but that everything will all pan out in the long run according to God's will and God's own time table.