ADO.NET is the underlying data access API for .NET Framework (much like JDBC in Java). It’s been around since the first release of .NET.

LINQ to SQL is a data access framework built on ADO.NET and new language features that makes SQL Server data available natively in the object oriented style of programming. LINQ (Language-Integrated Query) and thus allowing to query data using programming language’s own syntax
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Linq IN query will pull back a set of results from SQL that is within a given range. This range can be set manually, or can itself be a query. So if you have an eCommerce application and you want to know what products you have in a given user’s cart

SQL

SELECT * FROM Product WHERE ProductID IN (SELECT ProductID FROM mycart)

Linq

List<int> itemQuery = new List<int>() {1,2,3,4};

var myProducts = from p in db.Products
                where itemQuery.Contains(p.ProductID)
                select p;

Linq To Sql only constructs the query when the Enumerator is tripped