According to Catholicism, who are “the elect”?
In a nutshell, this term for Catholics refers to the Church Militant and the Church Triumphant.
Here follows what the Catholic Encyclopedia has to say on the subject of the Elect:
Denotes in general one chosen or taken by preference from among two or more; as a theological term it is equivalent to "chosen as the object of mercy or Divine favour, as set apart for eternal life". In order to determine the meaning of the word more accurately, we shall have to study its usage both in the Old Testament and the New.
The Old Testament
The Old Testament applies the term elect, or chosen, only to the Israelites in as far as they are called to be the people of God, or are faithful to their Divine call. The idea of such an election is common in the Book of Deuteronomy and in Isaiah 40-66. In Psalm 104:6 and 43 and 105:5, the chosen ones are the Hebrew people in as far as it is the recipient of God's temporal and spiritual blessings; in Isaiah 65:9, 15 and 23, they are the repentant Israelites, as few in number "as if a grain can be found in a cluster" (ibid., 8); in Tob., xiii, 10, they are the Israelites remaining faithful during their captivity; in Wisd., iii, 9, and iv, 15, they are God's true servants; in Sirach 24:4, 13 and 46:2, these servants of God belong to the chosen people.
The New Testament
The New Testament transfers (excepting perhaps in Acts 13:17) the meaning of the term from its connection with the people of Israel to the members of the Church of Christ, either militant on earth or triumphant in heaven. Thus 1 Peter 1:1, speaks of the elect among the "strangers dispersed" through the various parts of the world; 1 Peter 2:9, represents them as "a chosen generation, a kingly priesthood, a holy nation, a purchased people", called from darkness into God's marvellous light. St. Paul, too, speaks of the elect (Romans 8:33) and describes the five degrees of their election: they are foreknown, predestined, called, justified, and glorified (loc. cit., 29, 30). He returns to the idea gain and again: 2 Thessalonians 2:12 sq.; Colossians 3:12; Titus 1:1-2; 2 Timothy 2:10. St. John gives the title of elect to those who fight on the side of the Lamb against the powers of darkness (Revelation 17:14). According to Luke 18:7, God hears the cries of his elect for vengeance; according to the first two Evangelists he will shorten the last days for the sake of the elect (Matthew 24:22, 24, 31; Mark 13:20, 22, 27).
If it be asked why the name elect was given to the members of the Church Militant, we may assign a double reason: first, they were freely chosen by God's goodness (Romans 11:5-7, 28); secondly, they must show in their conduct that they are choice men (Ephesians 4:17). In the sentence "many are called, but few are chosen", the latter expression renders a word in the Greek and Latin text which is elsewhere translated by elect (Matthew 20:16; 22:14). It is agreed on all sides that the term refers to members of the Church Triumphant, but there is some doubt as to whether it refers to mere membership, or to a more exalted degree. This distinction is important; if the word implies mere membership in the Church Triumphant, then the chosen ones, or those who will be saved, are few, and the non-members in the Church Triumphant are many; if the word denotes a special degree of glory, then few will attain this rank, and many will fail to do so, though many are called to it. The sentence "many are called, but few chosen" does not, therefore, settle the question as to the relative number of the elect and the lost; theologians are divided on this point, and while Christ in the Gospels urges the importance of saving one's soul (Luke 13:23, 24), he alternately so strengthens our hope and excites our fear as not to leave us any solid ground for either presumption or despair.
Simply put the Elect are those saved and are in heaven, thus forming the Church Triumphant and the Church Militant on earth.
What is the Church Militant?
The Church on earth, still struggling with sin and temptation, and therefore engaged in warfare (Latin militia) with the world, the flesh, and the devil.
And what is the Church Triumphant?
The Church of all those in heavenly glory who have triumphed over their evil inclinations, the seductions of the world, and the temptations of the evil spirit.