An antibiotic is a medicine which kills one specific type of pathogenic micro-organism (germ). This specific micro-organism is bacteria. Antibiotics are only effective on and approved for use to treat bacteria or to prevent a bacterial infection. An example of a bacterium that is common is Staphylococcus aureus ("Staph infection"). Different antibiotics work differently and on different types of bacteria, but the sole aim is to kill the bacteria causing the infection.
Antibiotics are not for treating infections caused by other micro-organisms, such as viruses (like a cold or flu) or fungi (like athlete's foot or the respiratory fungal infections). The term "Antibiotic" is only used for medicines to treat bacterial infections in today's lexicon.
Examples of a few common antibiotics are Penicillin, Z-Pak (azithromycin), and Cipro (ciprofloxacin), but there are hundreds in common usage. Before antibiotics, people could easily die from a toothache (abscess), appendicitis, or minor cuts that became infected.
More information:
Using antibiotics when ineffective or unnecessary enables the spread of antibiotic-resistant bacterial strains.
Antibiotics are not innocuous. They have toxicities and side effects. This means that antibiotics not only do no good in a viral infection, they actually can harm the patient. This is at odds with a fundamental precept of medicine: "First, do no harm".
Antibiotics kill the bacteria, therefore, they are called bactericidal agents. Some medicines only impair bacterial growth and are known as bacteriostatic agents or antibacterial agents. These are often used as topical medicines to treat a localized bacterial infection.
Another related term is anti-microbial which would encompass a larger group of microscopic pathogens including viruses and fungi, usually used to refer to those chemicals that kill germs on surfaces and objects, not used directly on the body.
More information re: antibiotics for viral infections:
