Essentially, the whole game becomes nullsec, without bubbles in HS or LS. Highsec is effectively nullsec, where it relates to aggression between the Corps involved in the wardec.
Anyone from the wardeccing Corp can attack any member of your Corp, freely anywhere, with no CONCORD interference and no sec status loss; and no protection from sentry guns either. Your Corp members can also attack the wardeccing Corp’s members (which will grow substantially in the next 24-48 hours).
While ever the wardeccing Corp is willing to pay the wardec fee, the wardec can be maintained forever.