Hallelujah! This is the only place in the New Testament where the word is used. The word is a combination of the Hebrew Hallal (meaning loud exclamation of praise) and Yah (which is a shortened form of God's proper name first given in Exodus 3). It is usually translated Praise the Lord. Curiously, Hallelujah is frequently used in the Psalms. The word is common in Psalms used around Passover (113-118) where the focus on on the punishment of the wicked and the exodus from Egypt.
We then get a great celebration in heaven that calls the church the "Bride of Christ". However in verse 9 the church is invited to the "marriage supper of the Lamb". So the church is the Bride of Christ AND the invited wedding guests at the feast. John conveys to us the greatest, most joyful, celebration humans can know. The uniting in love. Remember in earlier chapters on the destruction of Babylon that one thing that would be missing was the cry of the bride and bridegroom. The celebration of the joy of marriage, the continuation of life, the moving forward of the generations of humanity is cause for great celebration. We are invited, as Christ followers, to the greatest celebration . . . the celebration of the marriage feast between Christ and the church.
Now comes the "rider on a white horse". In biblical times when a king came in peace he rode a donkey (see Jesus on Palm Sunday) however, when a King came to conquer he rode a horse. The interesting piece about Jesus on the horse is that though he is "dipped in blood" his army stays clean. I believe this is meant to signify that the battle is his. Christians understand that the war against evil is a war that is already over. God has assigned the victor and it is Jesus. Through his death and resurrection, Jesus has already attained the final victory and through we may struggle from day to day, week to week, year to year; and though the church may suffer persecution and difficult trials in the meantime -- Jesus has won the victory. Notice also that for all the imagery of battle and war it is the sword out of his mouth (aka the Word of God) that wins the battle.
The beast (Antichrist = evil) and the false prophet (all false idolatrous religion) are defeated by the word of God and cast into permanent separation from God. What I said in the beginning applies here: remember that the entire book of Revelation can be summed up in two words: "Jesus Wins". That is what is being described here. In the war between good and evil, heaven and hell, the Antichrist and Jesus the King of Kings and Lord of Lords: Jesus wins.