The dictionary says the definition of newswire is
"a wire service that transmits up-to-the-minute news, usually electronically, to the media and often the public."
Understanding the definition of a newswire will also help you to better understand what a newswire is and what it does.

The term newswire came into existence during the time of the telegraph, a time when news could first be transmitted at speeds never seen before across the country from newspaper to newspaper service. The term was used in reference to telegraph wires and, although we don't really use telegraphs any more, the term continues to be an important part of the news industry today.
Today, the definition of newswire simply refers to a system of people and processes that is used to deliver the most recent up to the minute and breaking news across the country.
A newswire is responsible for supplying stories, graphics, and photographs to other actual newspaper companies that produce a product for consumers.
One of the most popular newswire service companies is the Associated Press. However, there are other well known newswire service companies as well including Reuters and Scripps-Howard.
The difference between a newswire and a newspaper service, even though they sound similar, is that a newswire does not produce its own product. For example, the Wall Street Journal is an actual newspaper produced by a news company, but the Associated Press, a well-known newswire, does not produce an actual product that is delivered to consumers.
The benefit of newswire services is that they have reporters that cover news all over the world.
Newswire companies make their profit from subscription fees. These fees are the ones paid by the local television and newspaper companies throughout the world. The subscription fee must be paid in order for these local news industries to have access to the stories, pictures, and graphics that are produced by the newswire companies.