Sterling silver is 92.5% pure silver blended with other metals to make it firmer and to help it hold it's shape. Still, it is the softest of the three types of trim. With exposure to air sterling will tarnish.
It is a thick layer of sterling silver laid on top of a firmer metal during the manufacturing process. This makes the finished piece firmer and less prone to bending. The silver layer is thick enough that it is engraved just like solid sterling and will also tarnish with exposure to air.
Plate is a very thin layer of silver electroplated on to a thick piece of base metal. It cannot be engraved. The engraving you see on silver plate trim is actually done on the metal underneath. The plate is so thin that you can still see the engraving.
Pure silver is just what the name suggests-- pure silver. However, in the pure form, silver is too soft to use in trim applications or even in jewelry. The only place you see it on horse gear is in inlaid bits and spurs. It is used for inlays because it can be hammered into the cuts and then engraved.
Each type of silver must be clearly marked, but it can be marked in several ways. Sterling is typically marked "solid sterling", "sterling silver", "sterling", or "92.5." This last mark is a reference to the 92.5% pure silver in sterling. Sterling overlay can be marked "overlay", "silver overlay", "sterling silver overlay" or even "92.5 overlay." Plate is marked "plate" or "silver plate." It is against the law to incorrectly mark silver content.
Both sterling and sterling overlay must be cleaned with some type of silver polish in order to remove the tarnish that develops over time.
Silver plate must never be cleaned with silver polish. It is always treated with a laquer to resist tarnish. Because the layer of silver is so thin, the abrasives in silver polish will wear through the laquer and even through the electroplated silver. Plate should always be cleaned with warm, soapy water and a soft cloth.
The amount of silver used to make silver plate trim is so small that it can be sold much more cheaply than either sterling silver or sterling overlay.