A - Thanks for your question!
I will take on the second question first. A sacramental is defined by the Catechism of the Catholic Church as:
1667 "Holy Mother Church has, moreover, instituted sacramentals. These are sacred signs which bear a resemblance to the sacraments. They signify effects, particularly of a spiritual nature, which are obtained through the intercession of the Church. By them men are disposed to receive the chief effect of the sacraments, and various occasions in life are rendered holy."So, a sacramental is a sign that helps us to receive the grace that comes through the Sacraments of the Church. They are another help that God gives us to draw us further into holiness. Some common sacramentals include holy water, common blessings (blessing of a house, blessing of a child by a parent, etc.), Sign of the Cross, blessed medals, rosaries, etc. No Sacramental is considered "magic" or "holy" in and of itself. The purpose of them is to bring us closer to God. They do nothing without our decision to accept God's grace and then grow in holiness.
Furthermore, medals, scapulars and other objects worn on a person are a reminder to us of prayer and devotion to God. For an object, such as a medal, to be considered a sacramental then it must be blessed. Otherwise they are considered mere objects or jewelry.
Now, as for scapulars - they originally were large pieces of clothe worn over the shoulders to signify taking on the yoke of Christ. They then grew smaller and are now worn like necklaces around the neck with only small pieces of cloth (and now some are medals). They can be worn as a way to associate yourself with a religious order or as a devotional. There are certain promises associated with wearing a scapular, in particular that the Virgin Mary will assist the wearer at the time of death. Again, this isn't some kind of "get out of jail free" card. We must certainly do our part to grow in faith, hope and charity. IF we do that, then the promise will be fulfilled. But, this is a two-way street and both parties must do their part.
There are many different kinds of scapulars (more than 20). As briefly mentioned above, there are some scapulars that are associated with particular religious orders. So, some scapulars require that a person is affiliated in some way with that particular order(e.g., the scapular of the Most Blessed Trinity - Trinitarian Order). The most popular kind are the scapular of Our Lady of Mount Carmel (affiliated with the Carmelites) called the brown scapular and the scapular of the Immaculate Heart of Mary is called the green scapular.
Saint Pius X allowed the use of a medal instead of the traditional cloth scapular in 1910. But, to be enrolled or invested into the scapular devotion one must first use the traditional scapular, which can then be replaced by the medal one. Here is the text of the statement by Pius X.
For more info on all of this go to the following links:
More on Sacramentals
So, to specifically answer the first question, the scapular medal is used as a sacramental associated a particular devotion to Christ and Mary as explained above and other medals are generally worn by someone who has a particular devotion to a certain Saint or devotional that the medal is associated with. Some of the more common medals include the miraculous medal and the Sacred Heart medals.
More on devotional medals