The Yuzhaan Vong were extragalactic aliens that were highly resistant to the Force. They waged a long, bloody war against the Republic that played out over the New Jedi Order novel series.
The war began in 25 ABY, 21 years after the events of Return of the Jedi, and ended in 29 ABY.
The complete list of New Jedi Order materials can be found on Wikipedia. In in-universe (more or less) chronological order:
- Star Wars: Invasion (2009-2011 comic book series)1
- Vector Prime (1999 novel)
- Boba Fett: A Practical Man (2006 e-book)
- Dark Tide I: Onslaught (2000 novel)
- Dark Tide II: Ruin (2000 novel)
- Agents of Chaos I: Hero's Trial (2000 novel)
- Agents of Chaos II: Jedi Eclipse (2000 novel)
- Balance Point (2000 novel)
- Emissary of the Void (2002 short story)
- Recovery (2001 e-book)
- Edge of Victory I: Conquest (2001 novel)
- Edge of Victory II: Rebirth (2001 novel)
- Star by Star (2001 novel)
- Dark Journey (2002 novel)
- The Apprentice (2002 short story)
- Enemy Lines I: Rebel Dream (2002 novel)
- Enemy Lines II: Rebel Stand (2002 novel)
- Traitor (2002 novel)
- Destiny's Way (2002 novel)
- Ylesia (2002 e-book)
- Force Heretic I: Remnant (2003 novel)
- Equals and Opposites (2004 comic book)
- Force Heretic II: Refugee (2003 novel)
- Or Die Trying (2003 short story)
- Force Heretic III: Reunion (2003 novel)
- The Final Prophecy (2003 novel)
- The Unifying Force (2003 novel)
1 Nod to Thunderforge for pointing this one out