The Nine Levels of Prayer

—Purgative Way—
1. Vocal Prayer
2. Mental Prayer
3. Affective Prayer
4. Acquired Recollection
Bridge: Dark Night of the Senses
—Illuminative Way—
5. Infused Contemplation
6. Prayer of Quiet
Bridge: Dark Night of the Soul
—Unitive Way—
7. Simple Union
8. Conforming Union
9. Transforming Union

For more information and details, see this post here. Notice the lowest form of prayer is Vocal Prayer, meaning using words either audibly or mentally, whereas mental prayer is primarily in the intellect, focusing on God or meditating on some mystery without words of any kind. The Holy Rosary combines the first two levels through vocal prayers while meditating on the mysteries of our Lord (Mental Prayer). The saints tell us that Mental Prayer is the gateway to all the higher forms of prayer, and the higher you ascend also corresponds to your progress in virtue. Our Lady of Fatima requested 15 minutes of mental prayer for the First Saturdays devotion. St. Alphonsus recommends making 15 minutes of mental prayer each day. See the resources page on prayer. The best book of meditations I have found is Fr. Ignatius’ School of Jesus Crucified (1866), available for free with this app.