A bacteriophage also known informally as a phage is a type of virus that infects bacteria. In fact, the word "bacteriophage" literally means "bacteria eater," because bacteriophages destroy their host cells
William Twort was the first who discover bacteriophages in 1915 and two years later Felix d´Herelle realized that they had a potential to kill bacteria. Karolinska Institute has decided to award the Nobel Prize in Physiology or Medicine for 1969 jointly to Max Delbrück, Alfred D. Hershey and Salvador E. Luria for their discoveries concerning "the replication mechanism and the genetic structure of viruses"  .
Most bacteriophages present an icosahedral proteinaceous head, which contains the nucleic acid, either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Exceptions to this rule are few cases where a lipid envelope forms part of the head, and those other cases where the head presents a filamentous geometry. The way bacteriophages infect the host cell is the basis of a main difference among them: one group (Caudovirales) has a specialised structure (the tail) that
is responsible for the recognition of the host cell and the viral genome delivery, whereas those bacteriophages without tail present a variety of infecting strategies
. Phages are classified by the International Committee on Taxonomy of Viruses (ICTV) according to morphology and nucleic acid.
Phages have various possible life cycles which, along with interaction with their physical environment, dictate their role in bacterial/archaeal biology. The lytic life cycle is where phages infect and rapidly kill their infected host cells, thereby shaping bacterial population dynamics and occasionally assisting in their long term evolution via generalized transduction. The lysogenic life cycle in contrast, is where phages instead of directly killing their hosts, integrate into their host genome, or exist as plasmids within their host cell. This lysogenic life cycle can be stable for thousands of generations and the bacteriophage may alter the phenotype of the bacterium by expressing genes that are not expressed in the usual course of infection in a process known as lysogenic conversion .