Safety is the prevention of accidents (accidents which may or may not involve human agents, but are in any case not intentional).

Security is the prevention of malicious activities by people (mugging, burglary, robbery, terrorist activities, etc.).

So safety may include activities such as clearly marking pits and holes on a building site to avoid people accidentally falling into them. Or putting up warning signs to warn people that there is high-voltage equipment past a certain point and the general public should stay the hell away.

Security may involve things such as the placement of heavy lorries across access roads of farmers’ markets to stop assholes from running their cars or vans into crowds of people. Security involved checking passengers and their luggage for forbidden items which could be used to attack others in flight.