Summoner

A Summoner is a magic user who is able to summon various magical/supernatural beings to aid them in daily life and/or combat. Because of how difficult and taxing summoning can be, Summoners aren't as proficient in spellcasting like other magic users but really when you can summon a raging minotaur or fire elemental to help you out, it does balance it out quite nicely.

Most fantasy settings like some form of summoner present or at least the skills to use summoning in battle.

There was a cool PS2 RPG of the same title which was the shit when it came out. Go on, go and look for it if you are unaware of it's existence, it will give you a good idea of what a Summoner is.

Daemon summoning is often seen as a sub-class of general summoning in of itself, due to range of daemons and the various dangers daemon summoners face when dealing with the abyss.

Summoning generally is shown to have it's own downsides though. It is cool to try to summon an elemental but if you are not strong enough to control it or you are not careful enough in the rituals the summons can break free and quickly make life unpleasant for you, from eating you or controlling you in turn. Like spells as well summoning can take time and concentration, leaving you open to attack. Guess nothing is really for free.

Pathfinder

In the pathfinder setting, Summoners are an intuitive spellcasting class with a focus on buff spells and the same magic progression as Bards. They swap Bard-song for being the most adept magic users in their setting at the art of summoning, and in particular they will form bonds with beings known as Eidolon, which makes them a pet class. Over time the souls of the summoner and the Eidolon overlap until they share a piece of each other's souls. While a powerful entity to aid them, the summoners often lack in power and abilities in more general spellcraft. However, they can be built up to near-CoDzilla levels if you focus exclusively on making them into a "super solo" and fighting alongside their eidolon if you don't mind blowing the rest of the party off completely.

The Classes of Pathfinder 1st Edition
Core Classes:	Barbarian - Bard - Cleric - Druid - Fighter - Monk Paladin - Ranger - Rogue - Sorcerer - Wizard
Advanced Player's Guide:	Alchemist - Antipaladin - Cavalier Inquisitor - Oracle - Summoner - Witch
Advanced Class Guide:	Arcanist - Bloodrager - Brawler - Hunter - Investigator Shaman - Skald - Slayer - Swashbuckler - Warpriest
Occult Adventures:	Kineticist - Medium - Mesmerist Occultist - Psychic - Spiritualist
Ultimate X:	Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante