A query is a request for information from a database which matches a set of criteria.
You use queries all the time when you use a search engine such as Google. You type in the term that you want some information about and click 'search'. The search engine looks through its database and displays a list of results which match the term that you used.
If you are setting up a query directly to a database, for example the coding that goes on behind the scenes many web sites, then the developer will most likely use a formal language called 'Structured Query Language' or SQL to set up the query.
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Click on this link: Database Query