My theology professor has lectured on the divine economy, or work of the Holy Trinity, and the operation of grace, so I thought I'd share this with you.
Grace literally means a gift and in its widest sense any divine gift. It's not acquired through our own efforts or deservance. More specifically it refers to God's gift of union with Himself, with forgiveness for our sins.
Salvation is sola gratia, by grace alone, as compared to the sola fidei or sola Scriptura of the Protestants (as Fr. Leger says, heretics). This is made abundantly clear in the New Testament and repeated in the works of the theologians across the centuries.
There are two kinds of grace that the professor highlighted for now, uncreated and created grace. I know, what an odd-sounding statement. Uncreated grace refers to God sending himself into our souls to dwell and to act there. Usually we think of the Holy Spirit, for instance in the grace of faith that moves the will to believe in God. But God in all three divine hypostases is present, for where One is present the other Two are in what is known as perichoresis or circumcessio, the total and perfect mutual indwelling of the Most Holy Trinity.
Created grace transforms us and unites us with God. It leads us to become sons and daughters of God and to live the life of the theological virtues of faith, hope, and charity.