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.
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.