Delver is a tempo deck archetype named after Delver of Secrets. These decks deploy an inexpensive threat and then protect their advantage with cantrips, counterspells, removal, and other cheap interaction rather than trying to dominate a long game.
The name has outgrown the original card, and some Delver-style decks don't actually play Delver of Secrets anymore. Threats such as Dragon's Rage Channeler and Murktide Regent can fill similar roles alongside efficient spells like Force of Will and Lightning Bolt.