Michael is one of the 24 archangels. The only archangel mentioned in the Bible. He specifically is for something, like all believer's have their own guardian angel. Then you are wondering what are these 24 archangels.

“But Michael the archangel, when contending with the devil he disputed about the body of Moses, durst not bring against him a railing judgment, but said, The Lord rebuke thee” (Jude 9). Why? Because at one time God had made Lucifer the chief of the archangels; and Michael, being one of the archangels, had been under his authority. Later Michael, in obedience to God, sought the body of Moses because one day Moses was to be raised from the dead temporarily (possibly at the Mount of Transfiguration). When Michael was hindered by Satan, he could, in a rebellious spirit, have dealt with that rebellious one, Satan, by opening his mouth and reviling him. But he dared not to do so. All he said was, “The Lord rebuke thee.” (With men it is a different story, since God has never placed men under the authority of Satan. Though once we fell into his rule, we were never under his authority like the angels are with each other.)

The archangel Michael comes to help in Rev. 12 for Israel. According to Daniel 12.1, at the time of trouble Michael shall stand up for the children of Israel.

Besides the angels, who are entitled to sit on thrones and wear golden crowns ahead of the Lord Jesus? God had originally appointed angels to govern the universe. But one of the archangels fell and turned himself into Satan, there thus coming into existence the satanic kingdom. As to those angels who had not followed Satan in rebellion, God still assigns them the rule over the universe. Now just as Michael is the chief prince over the nation of Israel (Dan. 10.13), even so, all of us who are redeemed have our guardian angels (Acts 12.15; Matt. 18.10; Heb. 1.14). The 24 elders sit while the seven angels who blow the trumpets stand before God (8.2). They are now in charge of the universe. When they see people getting saved they are not jealous at all; rather, they praise God for it. They will govern the universe until the kingdom shall come; and then they will resign their appointments and there will be the transfer of the government of the universe to men (Rev. 11.16-18; Heb. 2.5-8). This is why there is no mentioning of the 24 elders after 19.4.

Proof of the 24 archangels: http://www3.telus.net/trbrooks/Revelation_4.htm

1st rapture is part of the man-child (Rev. 12.5). War in heaven occurs (Rev. 12.7) as the man-child's coming before the throne cause the archangel Michael (and angels) to war against Satan (Rev. 12.7) for there can be no room for the raptured overcomers and Satan in heaven at the same time. Satan accuses the brethren day and night before God in heaven during the first three and half years (Rev. 12.9-10) before Satan is finally cast down at the 5th woe (Rev. 9.1-12) that lasts 5 months as the first part of the Great Tribulation (Rev. 9.10).

Jude records the word which Michael the archangel declared to Satan: "The Lord rebuke thee" (v.9). After that word was spoken Satan did not dare resist anymore. We may therefore use this same word in our prayer against him. We ask the Lord to rebuke the enemy. We ought to know that the Lord hearkens to such prayer. If we ask Him to rebuke, He will rebuke. We must also believe that after the Lord has rebuked Satan, the enemy is no longer able to withstand, for he is afraid of the Lord’s rebuke. When our Lord rebuked the wind and the sea, these elements listened to Him and instantly the wind ceased and the sea became calm. His rebuke produces the same effect on Satan.

Lucifer was one of the archangels but then made himself Satan. I PROPOSE MICHAEL WAS ASSIGNED BY GOD TO REPLACED LUCIFER AS ONE OF THE 24 ARCHANGELS TO MAINTAIN THE SAME NUMBER: 24 ARCHANGELS. 24 is the number of 'administration' like the 24 courses in the OT.