Who are the 144,000 in Revelation?

The identity of the 144,000 in the Book of Revelation have raised many speculation.

Here it's a list of the possible identities:

One view holds that the reference is to be taken literally to refer to ethnic Jews, the 12 tribes of Israel. This is taken to support the notion that God’s purposes during the Great Tribulation will be focused on ethnic Israel. This view maintains that the 144,000 are to be distinguished from the multitude from many nations. The other position maintains that the reference to the 144,000 is symbolic and pertains to believers (not merely Jews) who are alive during the Great Tribulation and that the 144,000 are to be identified with the great multitude.

The guys at B & H Blog Academic have published a short article on this issue, and it would be a good thing for you to read it as well.