Antibiotics are medications used to treat, and in some cases prevent, bacterial infections.

They can be used to treat relatively mild conditions such as acne as well as potentially life-threatening conditions such as pneumonia. 

However, antibiotics often have no benefit for many other types of infection and using them unnecessarily would only increase the risk of antibiotic resistance, so they are not routinely used.

 http://primarycare.hscni.net/pdf/I_dont_cure_the_cold_antibiotic_leaflet.pdf