In verse one John is told what must happen "after this". He just finished with the letters to the churches, so "after this" means after the church age. From this point on, the church is never mentioned except in heaven. This is another reason I believe the rapture happens before the Tribulation. Besides, the Tribulation is the culmination of God’s dealings with the nation of Israel (see Daniel 9:24-26 and 12). In Jeremiah 30:7 it is called the time of Jacob’s trouble (Jacob is Israel).

Now John is given a vision of God’s throne in heaven. Biblical prophets were typically shown a vision of God’s glory before being given a prophecy. There is obviously a lot of symbolism here, and we could spend years speculating on their meaning. But notice the reference to lamps in verse five. Here they represent the Holy Spirit. Maybe the lamps representing churches back in chapter one signify the fact that where God’s people are, there is his Spirit (see Matthew 18:20). Now Jesus, the Lamb, is ready to execute judgment on the earth. It couldn’t begin while Christians were still on the earth for at least two reasons: God would not judge his own Spirit, and Jesus already paid for our sins, so leaving us here to suffer judgment would mean he suffered twice for the same sins.

Seal #1: the white horse
The rider on the white horse could represent the Antichrist, since he is "bent on conquest". Christ rides a white horse later, so riding on a white horse could signify the fact that the Antichrist is impersonating Christ. The fact that he carries a bow but no arrows could mean that he poses the threat of war but doesn’t wage it yet, or it could signify some type of missile warfare since the bow is a weapon of distant combat.

Seal #2: the red horse
This rider carries a sword, possibly signifying ground or hand-to-hand combat. Red is the color of blood, meaning war is waged and not just threatened.

Seal #3: the black horse
Black is the color of death, which follows logically after war. Another result of war is famine, signified by very expensive basic necessities.

Seal #4: the pale horse
I couldn’t find any comments on what the pale color could mean, but only on the two riders. Death concerns the physical body, and Hades concerns the spirit. They were given control over only 1/4 of the earth, so their destructive powers are still being limited by God. As mentioned in seal #3, famine naturally follows war. Plague is another result, since medical help will be scarce and living conditions will be unsanitary. We also know that modern medicine can’t stop everything, and many bacteria are resistant to known treatments. It is apparent that God allows the wild beasts of the earth to leave their secluded homes in forests and jungles to attack man. This could also be the result of war, since the animals’ habitats might be destroyed.

Seal #5: souls under the altar
The souls ask God for vengeance against those who had killed them. Notice that they didn’t criticize God for allowing them to be martyred, and they didn’t seek their own vengeance. We would do well to remember this when we face trials. The souls are told to wait until the total number of those who would be killed as they were was reached. This shows the absolute control God has over the forces of evil, since he limits the number they can kill. By the way, God didn’t rebuke the martyred souls for asking for vengeance; he only told them to wait.

Seal #6: the sky is falling!
War, famine, and plague are not enough to get people’s attention. Only after global and cosmic catastrophes occur do people finally admit that they are suffering the judgment of God. Even then they don’t repent of their evil deeds or their rejection of God; they just want to hide.

The lull before the storm
Think of how weird it will be when there is not the slightest breeze anywhere on earth. God pauses his judgment to seal his people, the Jews, so they will not be killed. The obvious contextual meaning of the groups of 12,000 each are the 12 tribes of Israel. Some cults try to make it mean other things because they think God abandoned Israel and replaced it with the church, but this is not Biblical.

While this "sealing" is happening on the earth, John is shown a multitude in white robes in heaven. He is told they are martyrs who came out of the great Tribulation. It is important when reading Revelation to remember that there are events in heaven happening at the same time as events on earth. You might wonder how the martyrs got to heaven when no mention is yet made of them being killed on earth, but remember that John is being shown a vision. God doesn’t have to wait till something actually happened to show it as being already done in heaven. Also note that the Greek seems to indicate that the action is continuing, that is, "these are they who are coming out of the great tribulation".  So these martyrdoms were still in the process of happening at this point.