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A query string is the portion of a URL where data is passed to a web application and/or back-end database. The reason we need query strings is that the HTTP protocol is stateless by design. For a website to be anything more than a brochure, you need to maintain state (store data). There are a number of ways to do this: On most web servers, you can use something like session state server-side. On the client, you can store via cookies. Or in the URL, you can store data via a query string.
On the world wide web, all URLs can be broken down into the protocol, the location of the file (or program) and the query string. The protocol you see in a browser is almost always HTTP; the location is the typical form of the hostname and filename (for example, www.techopedia.com/somefile.html), and the query string is whatever follows the question mark sign ("?").
For example, in the URL below, the bolded area is the query string that was generated when the term "database" was searched on the Techopedia website.