Summoners
A summoner is a human who can separate small amounts of mana from their body and control it at a distance. Little is known about the mechanics of this ability, but it seems to be hereditary - most children with at least one summoner parent are summoners as well.
In the past, summoners were seen as intermediaries between spirits and ordinary people. They were often chosen as oracles or heroes - instruments who would convey and execute the spirits' wills. As summoners became more common and the cultural emphasis on spirit worship diminished, their mysterious reputation gradually disappeared. In modern times, summoner abilities are mostly used to capture monsters for use in mundane jobs such as farming or delivery.
Spirit Pacts
A spirit pact is a special magical ritual that a summoner and a spirit can perform, to symbolize a friendship or formal agreement.
In the pact ritual, the summoner and spirit each offer up a small amount of mana, which will combine and solidify into a physical pact trinket, usually resembling a small mundane object such as a bell or crystal. The mana inside the trinket still "belongs" to the summoner and spirit even if separated from them, so it is inert; it cannot dissipate into the atmosphere or be used in a spell. However, it can be physically destroyed, which renders the pact null.
As long as the summoner keeps the trinket on their person, the pact makers can use it to keep track of each other. The spirit can sense the rough location of the trinket, and both can magically communicate to the other through it. The spirit can also use it as a teleport anchor - if a summoner is in great danger and calls for the spirit, the spirit may manifest out of the trinket and protect them. In fact, this is the origin of the word "summoner".
Pacts used to be an integral part of many religious traditions, but as time went on, spirits became more and more reluctant to trust individual humans with the responsibility of a pact. In present day, they're mostly seen as a historical curiosity.
Capturing Monsters
Pacts are a form of spirit magic, and humans have not yet found a way to replicate it without a spirit's help. However, a very simplified version of the same principle has been used for hundreds of years to capture and take control of monsters.
Technique
Monsters are made entirely out of mana. Therefore, if a summoner replaces enough of a monster with their own mana, they can override the monster's instincts and give it commands - essentially turning it into a mobile pact trinket. To capture a monster, summoners have to first attack a monster to lower its mana levels, then send their own mana into it. This is a difficult balance to strike; if too little mana is replaced the monster will simply be healed and resume attacking, while if too much is replaced its shape will destabilize and it will completely dissolve. But after a monster is successfully captured, the summoner can more or less control it indefinitely.
A captured monster can still act on its own if left without orders, as well as interpret moderately complex commands such as "protect me" or "don't let anyone enter this area". However, they're not any smarter than regular monsters, so dangerous misunderstandings are always possible. Some summoners prefer to take complete control of their monsters when they need them, and order them to stay still when they don't.
More powerful monsters require more mana to capture. A summoner's monster count is effectively limited by the amount of mana they can remotely control, though this limit can be increased with training and practice. The average summoner fresh out of school can manage about a dozen harmless monsters, or a couple more powerful ones.
Training & Employment
It takes years of study and practice to hone one's summoning abilities to the point where one can consistently capture and control monsters. However, usable monsters are always in such high demand that those who do are practically guaranteed steady employment.
Captured monsters obey orders unquestioningly, and do not need to eat or sleep - thus, they make very very efficient guards, mounts, and pack animals. A trained summoner may decide to use their monsters to supplement their own business, or rent them out to other people. A few traveling summoners use their monsters to essentially become one-man adventuring parties - their monsters attack while they cast healing and support spells from the back.
A variety of training facilities for summoners exist - some are integrated into mage academies or universities, while others are standalone. Like other educational institutes, these vary wildly in quality. A practical demonstration of your ability is usually a better indicator of skill than a diploma, so many summoners keep their most impressive monster close at hand like living trophies.