Quote from AmberK
You need a permissions plugin such as (in my order of preference):
Pex: http://dev.bukkit.org/server-mods/permissionsex - most versatile, best in my opinion.
bPerms: http://dev.bukkit.org/server-mods/bpermissions - easiest for newer people
Permbukkit: http://dev.bukkit.org/server-mods/permbukkit - has some hooks with Panels.