The Bride of Christ
I always thought of the bride of Christ as the church.
For the husband is the head of the wife, even as Christ is the head of the church: and he is the saviour of the body.
- Ephesians 5:23 (KJV)
Here we see that there is a strong parallel between the husband/wife, and Christ/church. It therefore makes sense that the bride (like the wife) is also the church.
But then someone pointed out this verse.
And I John saw the holy city, new Jerusalem, coming down from God out of heaven, prepared as a bride adorned for her husband.
- Revelation 21:2 (KJV)
So from that we can see that the bride of Christ is actually "New Jerusalem".
But will Jesus really marry a city?!?
No I don't think so. That would make no sense.
In the same way that church refers to people (not a building), I think that "New Jerusalem" refers to the Holy City with God's people in it.
In this way, both explanations as to who the bride of Christ is work.