A scapular is a sacramental that looks like two small pieces of wool cloth connected by string that is worn over the neck, either under or over one's clothing (typically under the clothing), such that one piece of cloth hangs over the chest, and the second piece of cloth hangs over the back. They derive from the scapulars which make up part of monastics' religious habits -- that ankle-length (front and back), shoulder-wide, apron-like part of the habit that basically consists of a long rectangular piece of material with a hole for the head (some of them have hoods and some had ties under the arms). Monastic scapulars came, over time, to be called jugum Christi (the yoke of Christ), and receiving the scapular (becoming "invested") took on solemn meaning. Abbreviated forms of the full monastic scapulars were to be worn even at night.
In addition to regular monastics of the First Order (i.e., friars) and Second Order (cloistered nuns), laity attached themselves to various religious orders, too, in what are called "Third Orders." Some lay members of Third orders -- "tertiaries" -- are "Third Order Religious" who live in a monastic community and generally take vows; most others are "Third Order Secular" who live in the world and generally make solemn promises. In the beginning, many of these lay people were invested with the full habit; later, they came to wear only the very small scapulars, as seen at left, under their clothing.
In addition to these Third Orders, Confraternities of lay-people (married or single -- just "regular Catholics") developed whose members were invested with Scapulars of Religious Orders to which they were attached. It is these scapulars for lay people belonging to a Confraternity or a Third Order that one generally thinks of when one hears the word "scapular."
Some scapulars have privileges and indulgences attached to wearing them, but like any sacramental (holy water, blessed candles, etc.), scapulars are not magic; their efficacy depends on the proper intentions and faith of the wearer. Only by following through on the promises one makes when becoming invested can the benefits associated with them be had. They are best thought of as signs of a commitment to do certain things and of one's being a part of a religious community. They act as reminders, too, of these things they signify and of the Saints who are parts of the religious community in question. They are reminders to behave with holiness.
How it Works and Where to get them
The first thing you need to do is to find out if enrollment in a particular Confraternity is necessary before wearing one with the rightful expectation of spiritual benefit. This varies with the type of scapular, but most scapulars do not require any sort of enrollment that your parish priest can't handle for you.
You can buy scapulars from Catholic Gift Shops, Catholic mail order catalogues, etc. They're very inexpensive, and you can also often find free ones from various places, such as the religious Order with which the desired scapular is associated or from charitable organizations and souls who make them available. Just do a Search for "Free Scapulars." Know, though, that free scapulars are often poorly made, are not made of wool, and are not of traditional design. It is best if you can find a traditional source for your scapulars, especially the Brown Scapular.
After you get your scapular, you must have it blessed by a priest. After it's been blessed, you then become "invested" when the priest recites certain prayers (different scapulars have different prayers for investement). Many scapulars do not require investment at all, but simply need to be blessed -- as do all scapulars -- and then used properly per the directions below.
You only need to have your first scapular blessed; it it wears out and you need to replace it, the blessing "transfers" to replacements. (The proper way to get rid of worn out scapulars -- or any sacramental -- is to either burn it or bury it.)
Scapulars can also later be replaced by a religious medal called the "Scapular Medal" (see picture at right), but if this is done, the new medal must be blessed. This medal must "show the image of Our Most Holy Redeemer, Jesus Christ, showing His Sacred Heart, and the obverse that of the Most Blessed Virgin Mary," according to a decree of Pope St. Pius X (see picture at right).
Below are some of the different types of scapulars. The religious Orders they are associated with and the date of the scapular's origin appear in italics under the Scapular's popular name.