I thought that this explanation of holiness by Ralph Martin in the article "Called to Holiness: Who, Me?" in Columbia Magazine was very insightful:
First, holiness is not a matter of how many prayers we say, how often we fast or how many activities we volunteer for - although all of these may have a relationship to holiness. According to the saints, holiness is not primarily a matter of external actions of piety or service but of the interior union of our will to God's will.
St. Teresa of Avila defined holiness as wanting what God wants; loving what God loves; desiring what God desires. Similarly, St. Therese of Lisieux said, "Perfection consists in doing his will, in being what he wills us to be." Toward the very end of her life, Therese said, "I do not desire to die more than to live; it is what he does that I love." Indeed, Jesus reveals that holiness is ultimately about growing in love - bringing our hearts, minds, wills, and bodies into harmony with God.