Raiding
Raiding is technically defined when you have 6 - 40 people in one raid group. The raid group is made up of 2 - 8, 5 person group. The raid group get experience for killing the mob or player when killed.

Why Raid
Certian content you need a raid group to complete, or need to be in to get credit for the kill.

In PVP you want to be in a raid group so each member of the raid group gets credit for the kill and some of the honor from it as well.

In PVE there are currently these raid instances (listed in order of dificulty)

• UBRS - 10 man
  • Resest with a new leader
• LBRS - 10 man
  • Resets with a new leader
• ZG - 20 man
  • Resets every 3 days
• Ruins of Anh'Qiraj - 20 man
  • Resets every 3 days
• Molten Core - 40 man
  • Resets every Tuesday during maintenance
• Onyxia - 40 man
  • Resets every 5 days
• Blackwing Lair - 40 man
  • Resets every Tuesday during maintenance
• Temple of Anh'Qiraj - 40 man
  • Resets every Tuesday during maintenance
• Naxxramas - 40 man (forth coming in 1.11)
  • Resets every Tuesday during maintenance