The IQueryable<(Of <(T>)>) interface is intended for implementation by query providers.
This interface inherits the IEnumerable<(Of <(T>)>) interface so that if it represents a query, the results of that query can be enumerated. Enumeration forces the expression tree associated with an IQueryable<(Of <(T>)>) object to be executed. Queries that do not return enumerable results are executed when the Execute<(Of <(TResult>)>)(Expression) method is called.
The definition of "executing an expression tree" is specific to a query provider. For example, it may involve translating the expression tree to a query language appropriate for an underlying data source.
The IQueryable<(Of <(T>)>) interface enables queries to be polymorphic. That is, because a query against an IQueryable data source is represented as an expression tree, it can be executed against different types of data sources.
The static (Shared in Visual Basic) methods defined in the class Queryable (except for AsQueryable, ThenBy, and ThenByDescending) extend objects of types that implement the IQueryable<(Of <(T>)>) interface.
For more information about how to create your own LINQ provider, see Walkthrough: Creating an IQueryable LINQ Provider in the MSDN Library and LINQ: Building an IQueryable Provider on MSDN Blogs.