LINQ api provide a way to access Object Data, Relational Data, and XML Data. System.LINQ defines more than 50 operators(also called standard query operator and can also be extendable) which are similar to SQL operators. Static type checking allows the assignment of selected result in correct IEnumerable object.
It was also misunderstood in early days of LINQ concept that only database is accessible from API. LINQ can used with any data object which are in main memory. It is really easy to use LINQ and filter item in list instead of writing foreach code and then apply if else conditions.
LINQ to SQL translates query expression into T-SQL It is simple ORM.
Some cool facts about LINQ.
LINQ query does not execute until it is actually use/access by code. This concept is called Deferred Execution. So most operators are lazy.