"Jund" is the red-black-green shard from Shards of Alara.
More importantly, however, it's a term for a midrange archetype in those colors, using a mixture of quick disruption, multi-faceted removal, and powerful creatures, with lots and lots of 2-for-1s, via cards like Bloodbraid Elf, Hymn to Tourach, or Thragtusk. This is clearly a powerful strategy since we've seen periods where Jund decks were ascendant in Standard, Extended, Modern, and Legacy — often multiple formats at the same time!
Magic pros have said things like, "Jund is at best 55% and at worst 45% against everything." Still, having a "55%" matchup against Jund is much better than a "45%" matchup; and when Jund decks are powerful, it's useful to at least have a good sideboard plan against the deck.
For example, the Jund Standard decks at Pro Tour Gatecrash featured these elements:
- Hand disruption: Liliana of the Veil, Rakdos' Return, Duress
- Resilient threats: Thragtusk, Garruk, Primal Hunter
- Powerful removal: Dreadbore, Abrupt Decay, Victim of Night, burn spells
- Plentiful sources of card advantage: Thragtusk, Huntmaster of the Fells, Olivia Voldaren, Mizzium Mortars, Rakdos' Return
- Incidental lifegain on cards that are great even without it: Thragtusk, Huntermaster of the Fells
- Aggressive utility lands: Kessig Wolf Run (compare to the plentiful use of manlands in other formats)
- Occasional mana acceleration: Deathrite Shaman, Farseek, Arbor Elf
What kinds of cards and strategies are particularly strong against Jund?
("Jund" is a broad category of decks, but it is a very clear archetype. Feel free to focus your analysis on one format, e.g. Standard.)