Who are the 144,000 and this great multitude?

Rev 7:3-5 The angel said, "Do not harm the earth, the sea, or the trees, until we mark the servants of our God with a seal on their foreheads." (4) And I was told that the number of those who were marked with God's seal on their foreheads was 144,000. They were from the twelve tribes of Israel, (5) twelve thousand from each tribe: Judah, Reuben, Gad, Asher, Naphtali, Maneh, Simeon, Levi, Issachar, Zebulun, Joseph, and Benjamin.