The modern (and also post modern) use of the term “contemplation” usually refers to thinking about something. Thus for example, a person might be said to contemplate marriage. In another instance, I might tell you that I have been contemplating the size of the universe. Thus in contemporary use the term “contemplation” is used in a similar way as the term “meditation”. So it might be said that a person has been contemplating the universe, or meditating on the size of the universe, and these would be similar activities.
The premodern (or medieval) use of the term was different however. Catholic monks, nuns and mystics used the term contemplation to refer to prayer states (communion with God states) that were beyond words and thought. Contemplative prayer is thus contrasted with discursive (verbal) prayer. Verbal prayer (intercession; making requests; petition) is understandable and can be said to be in the realm of the mind. Of course, normal prayer is still spiritual, and still has elements that cannot be understood, but for the most part we would say we understand it and can communicate it.
Contemplative prayer, in contrast, does not use many words. It is, instead, a non verbal form of communion with God. Thus, the contemplative is describing an experience with God, not necessarily thoughts about God, or words spoken to God. Contemplation is often described as closeness to God. It may be said to be an experience of the presence of God. A contemplative might say they “hear” God or “see” God, but it would be a spiritual use of these terms; and thus they would not usually mean that they heard words or saw visions.
Contemplation is usually practiced using silence and solitude as a method. There is much to learn about distraction, detachment, recollection, dryness, darkness, perseverance, listening and discerning. It takes a long time to learn to quiet the inner regions and hear God in the silence.
Contemplation is associated with monasticism. The contemplative has a life style of prayer. This usually includes a life dedicated to prayer, a renunciation of the flesh, self-denial (and its heretical cousin ascetiscm). The contemplative wants to live for God. They are interested in renouncing all that stands in the way of seeing God. Likewise, the contemplative wants to see his/her own evil. They don’t want to live in denial or blindness, but sit quietly to hear God’s assessment of their spiritual state.
Contemplation is also associated with mysticism. Experiences with God can be life changing, but difficult to describe. Often, mystic poetry or mystic proverbs or mystic allegory are used to express these experiences.
To read in more detail about moving from meditation to contemplation see A Comparison of Meditation and Contemplation.