The Millennium refers to the "1000 year" reign of Christ referred to in The Revelation.
The pre-millennial view has Christ returning *before* the millennium.
The post-millennial view has Christ returning *after* the millennium. A common distinctive of this view is that we, by our efforts, bring about the kingdom on earth, thus preparing the way for Christ's return.
The a-millennial view is that the 1000 years is figurative and apocalyptic, not literal, but shares the view that Christ returns after it. A commin distinctive of this view is that Christ will establish his kingdom on earth (not us), that he brings it with him (so to speak) when he comes bearing the sword. In the mean time, we are pilgrims and exiles in waiting.
Again, it's more detailed than this short snippet, but that's a start.