Summoning is a magic that involves using elements within the earth, (for example, water and dirt) to transmute a separate entity. These new entities generally have a limited life span, and will usually decay within the first hour or so once casted. They are used for manual labor, combat purposes, and as a form of body guards.
Summoning Arts is in actuality, a combination of both magic, and alchemy. The first "summons" were originally alchemists trying to use components and elements to make a human. Though most of these experiments failed. It wasn't until later when Alchemists tried combining Magic with their work, using both Black and White arts to fuse their creations together. These experiments were a success, and the first summons were created. At first these were just simple creatures, usually no bigger then 3 feet tall, and most of them performed manual labor for their creators. It wasn't till later when Summons found military purposes, and almost immediately were used to support troops in combat. The first "Combat summons" took the form of ancient gods, using the elements that were associated with such gods.
As of late, there have been recent discoveries, suggesting that summoning has actually been used in ancient times, supported by the recent findings of many temples across the world, containing the instructions to create a Summon.