I've seen both terms (veneration and devotion) used when referring to the Catholic attitude toward Mary and the Saints. According to Catholicism, what is the difference between these two terms?
They're two different actions in Catholic theology.
A Catholic devotion is the rituals and customs of 1, worshiping God, or 2, honoring the saints. Examples of these are "The Way of the Cross", Praying the rosary, the Holy Face of Jesus and going on pilgrimage. These are not Catholic liturgy, which is public worship lead by the Priest and other important Catholic leaders (see: The Roman Rite)
Veneration is the act of honoring a saint, usually by means of Canonization but also through the use of devotions like pilgrimage to relics of a saint. An important distinction is that you can have a devotion to Jesus (ex. The Holy Face of Jesus), as you can use a devotion as a form of worship (Latria, worship of the Trinity) but you cannot venerate Jesus, because veneration is about honoring a someone not from the trinity whose life deserves recognition (Dulia)
If you want to read more about this subject, St. Thomas Aquinas wrote about all the nitty-gritty of Dulia and Latria in his SUMMA THEOLOGICA in Question 103
A person can worship God, but doesn't worship a saint, only venerates, which means to deeply honor, respect the memory of the saint, pray to them and even have faith in their supernatural powers from the other world, with the understanding that the saint is a servant of God. Devotion is veneration to a particular saint or figure. I have to say that many times the lines between worship and veneration are blurred to the point where people only pray to a particular saint and not to God. This is my understanding of these terms. I may not remember correctly.