An infection occurs when a parasitic organism colonizes a host organism and gains nourishment from the host.
The term is most often used in reference to a bacterial infection, where a harmful opportunistic bacteria begins to grow in or on some part of an organism's body.
The term is also used in reference to viral infections, even though viruses are not "organisms" in the strictest sense of the word.
Because the parasite needs an opening in order to enter the hosts body, the host is especially vulnerable if it has recently scraped or cut its skin, although this is not at all necessary for an infection to occur.
There are examples of bacteria growing in mutualistic rather than parasitic fashion. The digestive systems of most organisms, including humans, depend on the presence of beneficial bacteria in order to properly digest food, especially plant matter.
Infections are usually fought by the use of antibiotics, which kill the bacteria causing the infection. Penicillin is one well known example.