CONTEMPLATION:
It is in defining contemplation that you have a wide diversity of answers. Mostly I think this is because it is outside of our experience. I give you a cite if you are curious.
I have heard it said that prayer is when you talk to God and Meditation is when God talks to you. Could Contemplation then be when you talk to each other? It would have to be more like when good friends talk to each other with one mind. Some of the East describe contemplation as "to see God"4 But it most certainly is involved with and perhaps one goal of mysticism. I could only give you direct experience on the first two and believe that contemplation is another state of awareness entirely.
PRAYER:
Christians are taught to practice prayer quite a bit. One Christian cite outlines seven types of prayer. These are prayers of petition, confession, adoration, intercession, meditation (silence,) thanksgiving, and consecration.1 It is interesting that meditation is described as silence and a form of prayer. Because there is a clear distinction between God and Man there may be more reluctance to allow the possibility of meditation. Although some form of meditation may be involved in all religions.3
If you take my meaning it suggests that God can talk to anyone who meditates and this is problematic for any religion which has a hierarchical structure. For them God speaks to the appointed who speaks for God to the masses. Prayer for the masses is safe for religion, any religion not just Christianity. Meditation is more suited to mysticism than organized religious practice.
I had to travel East to learn more about Meditation. In the East, the relationship between God and Man is less distinct. Buddhism does not even believe in God. For them Enlightment is the ultimate goal.
You might meditate in a group (as in "world" meditations,) but its essence is individual and not group oriented. I suspect it makes a difference if one person asks for something or a whole church full ask for the same thing (even if only to the people asking.) Although like the East, the west has mystical branches of its religions, they are often marginalized and western mystics can come up hard against doctrines of the religion of which they are a part. If you accept Thomas Merton as a mystic then his struggles may be an example of this.5
CONCENTRATION:
My experience with meditation suggests that meditation is more a process of the absence of mind rather than the emphasis upon it. Wiki's definition talks about intense focus on one object.3 But my experience suggests that this is only a prelude to meditation. This prelude may be described simply as concentration. (Which you could have asked about separately.) For those learning to meditate the most difficult part is often to quiet the mind. It is easy to give up if you don't have initial success. The first time I began a practice of meditation it took me over 3 years to quiet the mind. Even then success was not always guaranteed.
MEDITATION:
My best analogy is that meditation is like listening. As in listening you can't be talking or thinking of your next response. Not, in any event, if you are truly listening to understand. Your focus must be upon listening. When you intently listen you must be quiet and still. Your focus is to block out all distractions, but it is in the absence of all else that meditation takes place. Even in the absence of concentration.
Yes, there are mantras (chants) and other vocal and visual techniques but although we might describe them as "meditation techniques" they are there to guide us to a goal. They are not that goal. If you want to "watch" someone meditating you might get some idea from Sufi dancing. (Whirling.)2 Or look at pictures of the sitting Buddha.
Prayer for me is still a confirmation of what I believe, but my real spiritual practice involves the constant listening of meditation. It is certainly not a practice for everyone. Contemplation is another goal off in some distant future.