On the day of the launch the official servers will reset?
yes as far as I know
The official word is that all official Servers will be wiped on May 8th for the release. It is up to Private Sever owners/admins.
Does that mean wiping characters as well? Or just buildings and structures?
TOTAL Wipe. Characters, levels, buildings, structures. The whole db is reset. Everyone starts fresh from the beginning.