Like the cracker usage of
own, only more so. Can be used as an verb, adjective, or possibly even a noun (although
pwnage is more suited to use as a noun).
Pronunciations vary; "pone" is favored at the 3drealms.com forums although I personally prefer something like "pwun" or "pw'n" (preserving the "w" sound).
v: Haha, I just
pwned you at UT2003!
adj: Wow, this new site is pwn!
n: Another excellent pwn by R4nd0md00d134!