There are seven seals, seven trumpets, and seven bowls. The numbered groups are sequential within each group. In other words, the seals follow one another consecutively, and the trumpets follow one another consecutively, and likewise the bowls. That is why they are numbered one through seven. But this does not necessarily mean that the groups follows another in succession.
Pre-tribbers (and from what I understand, pre-wrathers) believe each of these groups follow the previous group. However, I do not think this is correct. I think that each of the three series of seven judgments give a recapping and magnification of a portion of the tribulation which was covered in the previous group.
This type of recapping and magnification is eminent in the book of Daniel. For in Daniel, the general outline of future events is given first (in the dream of the statue with the head of gold, chest of silver, belly of bronze, legs of iron, and toes of iron and clay). Then later in Daniel, it elaborates on portions of that prophecy with a more detailed account (when Daniel has the vision of the leopard with wings and the bear). And then later on in Daniel, in chapter 11, Daniel gets into real specifics of what happens in the very last kingdom (when he describes what the King of Fierce Countenance does in minute detail).
I believe that the seven seals give an overview of the entire 7-year tribulation period, with Jesus' coming at the 6th seal (and a glimpse of the beginning of the Millennium in chapter 7). Then the seven trumpets recap probably the latter half of the tribulation. Then the seven bowls give a magnification of the very last days of the tribulation and the second coming.
The Seven Seals
The seven seals parallel the Jesus' endtime discourse in Matt 24. Many pre-wrathers have seen this parallel, but unfortunately, few have seen its significance in determining the chronology of Revelation. The sixth seal is the most important, describing the darkening of the sun and moon. This event steadfastly connects the sixth seals to two other definite events, the end of the tribulation (Matt 24:29), and the beginning of the "Day of the Lord" (Joel 2:31).
First Seal
Now I saw when the Lamb opened one of the seals; and I heard one of the four living creatures saying with a voice like thunder, "Come and see." And I looked, and behold, a white horse. He who sat on it had a bow; and a crown was given to him, and he went out conquering and to conquer. (Rev 6:1-2)
And Jesus answered and said to them: Take heed that no one deceives you. For many will come in My name, saying, 'I am the Christ,' and will deceive many. (Matt 24:4-5)
Second Seal
When He opened the second seal, I heard the second living creature saying, "Come and see." Another horse, fiery red, went out. And it was granted to the one who sat on it to take peace from the earth, and that people should kill one another; and there was given to him a great sword. (Rev 6:3-4)
And you will hear of wars and rumors of wars. See that you are not troubled; for all these things must come to pass, but the end is not yet. For nation will rise against nation, and kingdom against kingdom. (Matt 24:6-7)
Third Seal
When He opened the third seal, I heard the third living creature say, "Come and see." So I looked, and behold, a black horse, and he who sat on it had a pair of scales in his hand. And I heard a voice in the midst of the four living creatures saying, "A quart of wheat for a denarius, and three quarts of barley for a denarius; and do not harm the oil and the wine." (Rev 6:4-5)
And there will be famines, pestilences, and earthquakes in various places. (Matt 24:7)
Fourth Seal
When He opened the fourth seal, I heard the voice of the fourth living creature saying, "Come and see." So I looked, and behold, a pale horse. And the name of him who sat on it was Death, and Hades followed with him. And power was given to them over a fourth of the earth, to kill with sword, with hunger, with death, and by the beasts of the earth. (Rev 6:6-7)
For nation will rise against nation, and kingdom against kingdom. And there will be famines, pestilences, and earthquakes in various places. All these are the beginning of sorrows. (Matt 24:7-8)
Fifth Seal
When He opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the testimony which they held. And they cried with a loud voice, saying, "How long, O Lord, holy and true, until You judge and avenge our blood on those who dwell on the earth?" Then a white robe was given to each of them; and it was said to them that they should rest a little while longer, until both the number of their fellow servants and their brethren, who would be killed as they were, was completed. (Rev 6:9-11)
Then they will deliver you up to tribulation and kill you, and you will be hated by all nations for My name's sake. And then many will be offended, will betray one another, and will hate one another.... And unless those days were shortened, no flesh would be saved; but for the elect's sake those days will be shortened. (Matt24:9-10,22)
Sixth Seal
I looked when He opened the sixth seal, and behold, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became like blood. And the stars of heaven fell to the earth, as a fig tree drops its late figs when it is shaken by a mighty wind. Then the sky receded as a scroll when it is rolled up, and every mountain and island was moved out of its place. And the kings of the earth, the great men, the rich men, the commanders, the mighty men, every slave and every free man, hid themselves in the caves and in the rocks of the mountains, and said to the mountains and rocks, "Fall on us and hide us from the face of Him who sits on the throne and from the wrath of the Lamb! For the great day of His wrath has come, and who is able to stand?" (Rev 6:12-17)
Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light; the stars will fall from heaven, and the powers of the heavens will be shaken. Then the sign of the Son of Man will appear in heaven, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory. And He will send His angels with a great sound of a trumpet, and they will gather together His elect from the four winds, from one end of heaven to the other. (Matt 24:29-31)
Seventh Seal
When He opened the seventh seal, there was silence in heaven for about half an hour. (Rev 8:1)
The silence in heaven is because the Kingdom has come to earth. In other words, Jesus, the angels, and the saints have left heaven because the Second Coming has occured at the sixth seal and so they are on earth.
The Seven Trumpets
I think that the sixth seal, and the seventh trumpet are both descriptions of the second coming of Christ. It is difficult to determine exactly when the trumpets begin, but they probably cover the last half of Daniel's seventieth week which Jesus called "great tribulation."
I think that the two witnesses are an explanation of the 3.5 years during which the trumpets occur. They might be the ones who call down the plagues on the antichrist and his kingdom. For instance, we are told that they can turn water into blood, something that will occur at the second and third trumpets.
The seventh trumpet finishes the "mystery of God." This is a likely reference to the Day of the Lord, which is the theme of most of the prophets.
The Seven Bowls
The seven bowls of the wrath of God are selectively poured out on the Antichrist and the followers of his kingdom. Those who have God's seal upon them are exempt from these plagues.
Then I heard a loud voice from the temple saying to the seven angels, "Go and pour out the bowls of the wrath of God on the earth." So the first went and poured out his bowl upon the earth, and a foul and loathsome sore came upon the men who had the mark of the beast and those who worshiped his image. (Rev 16:1-2)
If Christians were not present, the statement that the plagues fell upon those with the mark of the beast would be redundant.



