I'm writing this as I explain it to you in TS, but here goes anyway...
Basically, it's vanilla, hardcore minecraft. No Bukkit, no mods, no creative. We tend to work as a team to create a home and gather resources. "The Goal" tends to vary, but has been such things as "Get to the Nether", "Get to the End", "Defeat the Dragon", "Find a Stronghold", etc.
BTW.. hardcore means, you die, you're banned for some period of time (right, guys?).