What is Query?

A query is a set of instructions that describes 

• what data to retrieve from a given data source (or sources) 
• and what shape and organization the returned data should have.

Execution of LINQ involve three operations

• Obtain Datasource
• Create Query Expression
• Execute Expression

I. Obtain Data Source

Earlier: The source data is organized logically as a sequence of elements of the same kind. 

• A SQL database table contains a sequence of rows.
• an ADO.NET DataTable contains a sequence of DataRow objects.
• XML file, there is a "sequence" of XML elements (although these are organized hierarchically in a tree structure). 
• Collection contains a sequence of objects.

WITh LINQ: the specific type and structure of the original source data is not important.  The application always sees the source data as an IEnumerable<T> or IQueryable<T> collection. 

II. Query Expression

• Query Expression can be used to query and to transform data from any LINQ enabled data source.
• A query expression must begin with a from clause and must end with a select or group clause.

II. Execute Expression

• A query is not executed until you iterate over the query variable in foreach statement.