Apocalyptic prophecy is difficult because, unlike second-century Jewish Christians, we've (mostly) lost the art of reading this sort of literature. Hence we attempt to take the numbers literally. For example, we might try to say, as Sola'lor has done, that the 144,000 mentioned there are an actual group of 144,000 individuals.
I'm not convinced that's the way to read it. Revelation assumes that you know and understand the previous 65 books. It makes reference to dozens upon dozens of biblical themes, and it does this indirectly. Let's look at the case of the 144,000 in context.
Revelation starts with John having a vision of Jesus, and Jesus gives him a message to the "7 churches that are in Asia" (modern Turkey). The churches are commended and scolded and promised rewards for amending their ways. This is followed by a heavenly worship scene in which all of heaven and earth worships "the one who is seated on the throne."
Then we have a vision of the martyrs asking God when they will be avenged, and God tells them to wait a bit, but they will be avenged. They and all of heaven worship "the one seated on the throne and...the lamb."
And now (chapter 6), six of seven seals are broken. We can only assume that the plagues represented by the seals are God's answer to the martyr's prayers. In chapter 7, the 144,000 are described as "servants of God" who have been "sealed", which is to say spared from the plagues, the wrath of God. They are described as 12,000 from 12 tribes of Israel. Now, where have we seen this kind of enumeration before? Well, we've seen this in the book of Numbers just before Israel begins the conquest of the Promised Land. I think it's highly likely, therefore, that the 144,000 are a quasi-military picture or metaphor of the New Testament church as a whole, not a subgroup of that church. Revelation here calls up a military sort of image for the church, but it is not a military in the traditional sense.
It's also worth noting that the numbers 1,000 and 12 (and the doubling of 12) are all numberologically significant. 12 is the number of completion, so part of the point is to say that all -- every last one -- of those who are sealed by God (owned by God) shall be saved (whatever that means, we need not go there for purposes of this post, I think).
Having the seal of god on their foreheads indicates ownership. God owns them.
There are two lambs in Revelation. One is Jesus, the Christ. The other is antichrist. One is the real article, the other a parody.