Antiseptics are antimicrobial substances ,they kill microorganisms such as bacteria, fungi and viruses. They are applied to tissues or skin that have a wound or opening in them to prevent the risk of infections. This is what your doctor would apply to your knee if you scraped it before bandaging it up.
Antibiotics on the other hand is a medicinal substance which is used to destroy microorganisms INSIDE the body through specific chemical reactions. This is the kind of medicine your doctor would prescribe when you have a fever caused by bacterial infection.