In Rev. 7 there is not much language describing them, but it does say that they are from the tribes of Israel: 12,000 from each of the twelve tribes (though instead of "Dan" we read "Mannasseh"). When the caucasian leaders of a Biblical-literalist American religion claim to be among them, remember that the Book of Revelation clearly identifies the 144,000 as Middle-Eastern Jews!
For those who accept a non-literal interpretation (allowing non-Jews to be among the sealed!), consider this explanation (paraphrased from Asimov's Guide to the Bible): In the language of the time, 1000 was the largest named number. Twelve carries the connotation of sacredness, as in the twelve tribes and twelve months in a year. So 1000 times twelve times twelve again represents an unimaginably large number, the largest number they could speak of, doubly increased by a number with sacred significance: the countless resurrected souls eternally praising the "Holy of Holies". 144,000 is not a limit to the number of those who can be saved, only a limitation of human language to express the countless number of the saved, using a poetic description fitting for the magesty of the scene depicted in this passage of Revelation.