An Abjurer is a form of Specialist Wizard in Dungeons & Dragons. As the name suggests, they are specialized in the magic school of Abjuration, which focuses on defensive and protective spells. Abjurers are typically the tankiest of wizards, layering so many protective spells on themselves and their allies that they are practically invulnerable. Also, being "the tankiest of wizards" isn't like being "the fastest snail;" abjurers can compete with fighters or barbarians for a party's tank slot, especially in 5e.
Abjurers make excellent team-players, because they can layer on all kinds of protective buffing spells that make members of the party better able to handle whatever threat is thrown at them. They are also superb for battling enemy spellcasters, because Abjuration covers many spells that nullify spell effects (nothing like a Protection from Fire spell to ward off enemy pyromancers) or outright cancel the protective and buffing spells of enemy casters.
The School of Incantation
Because Advanced Dungeons & Dragons was full of strange ideas, a wizard specialization appeared in the Forgotten Realms that took a new spin on the Abjurer. Called the School of Incantation, with its predominantly female practitioners being called "Incantatrixes" - males are "Incantatars", and the term "metamage" is referred to as a gender-neutral version of the title - this is a style of magic that revolves around studying and mastering the way that magic can be turned against itself. Incantatrixes are masters of the art of spells that affect the casting of magic - combining metamagic spells with disruptive spells, banishment spells, and shielding spells.
The School of Incantatation, and the Metamage specialist wizard, appeared in the second appendix of "The Cult of the Dragon", a Forgotten Realms splatbook that focused on the titular dracolich-promoting cult.
For further details, see the page Metamage.
5th Edition's Arcane Tradition
Abjurers are now the beefiest of the wizards, or at least they were till Bladesinger was printed. Their level 2 ability, Arcane Ward, lets them create a magical shield whenever they cast an abjuration spell (not cantrip, proper spell). This shield creates a pool of temporary hitpoints that soaks up incoming damage, and which can be healed by casting more abjuration spells. At level 6, they gain Projected Ward, which lets them use their reaction to protect a creature within a 30ft line of sight with their Arcane Ward. At level 10, Improved Abjuration gives the abjurer the ability to add their Proficiency bonus to any ability checks they have to make whilst casting abjuration spells. Finally, at level 14, they gain Spell Resistance, which in this edition works out as Advantage on saving throws vs. spells and Resistance to spell-inflicted damage.