The Church Triumphant - those who have already earned their Heavenly Reward
The Church Suffering - those who are suffering the hell-fires of Purgatory, purging themselves of all the temporal punishment due to the venial sins they did not make up for on earth
The Church Militant - those on earth who are in the Battle of the Lord, in the state of Grace, working for the Reign of Christ.
I had read a lovely little except in my Missal regarding Christ the King that I find fitting for the Church Militant in a wonderful little explanation:
The royalty of Christ rests upon a twofold basis. He is our King by right of birth and by right of conquest. The first refers us to the personality of the Son of God, whereby, in His divine nature as God and by virtue of the hypostatic union, He is the sovereign Lord and Master. The second places before us the God-Man coming down on earth to rescue fallen man from the slavery of Satan, and by the labors and sufferings of His life, and passion, and death to win a glorious victory for us over sin and hell.
Viva Cristo Rey!