Put simply, antibiotics are drugs that either kill bacteria or stop them multiplying.
There are many synonyms used for drugs that fulfil these functions, such as antibacterials and antimicrobials. Strictly speaking, each term refers to drugs with slightly different activities and structures.
However, for the purpose of this discussion, the term antibiotic will be used to refer to any drug that has activity against bacteria.
These distinctions can become more important when one remembers that there are also drugs that are able to kill parasites (antiparasitic agents), drugs that stop viruses multiplying (antivirals), and drugs used to kill fungi (antifungals). It is important to realise that most antibiotics are only active against bacteria (although some are also active against certain parasites). Antibiotics have no effect against viruses.