What are the seals in Revelation?

A scroll with 7 seals (wax imprints that seal the scroll shut) is handed to the Lamb, Jesus Christ in Revelation 6. As each one is opened, something dramatic happens.

Seal One - a rider on a white horse conquering other leaders
Seal Two - a rider on a red horse bringing war
Seal Three - a rider on a black horse bringing economic instability
Seal Four - a rider on a pale horse bringing disease

These Four Horseman (of the Apocalypse as they are known) appear in another form in Zechariah 1. They represent angels bringing judgment to the world.

Seal Five - martyrs cry out for justice.
Seal Six - a great physical devastation of the earth:
  • Earthquake
  • Sun turns black
  • Moon turns red
  • Stars fall from the sky
  • Sky rolls up 
  • Every mountain and island removed
  • Kings of the earth go into hiding, yelling at the mountains and rocks.
Seal Seven - silence, rest.

Question: Are these things that happened, will happen OR are they statements regarding judgment (not specific to an event, but a statement of judgment against the things they represent)
  • Seal 1 - political leaders
  • Seal 2 - war
  • Seal 3 - economy
  • Seal 4 - health
  • Seal 5 - injustice
  • Seal 6 - the physical earth
Question: Could these events really, literally happen to such a degree? Could the sun turn black and anything survive? Could the stars fall from the sky? Could every mountain and island be removed?

The seals represent a dramatic prelude to a dramatic conclusion.  Most likely they represent a "judgment" on different areas of our world and could not be specific events that happened, or will happen.  These judgments happen all the time.