IEnumerable vs. IQueryable

IQueryable

• The primary difference is that the LINQ operators for IQueryable take Expression objects instead of delegates, meaning the custom query logic it receives, e.g., a predicate or value selector, is in the form of an expression tree instead of a delegate to a method.
• IQueryable extends the IEnumerable interface, so anything you can do with a "plain" IEnumerable, you can also do with an IQueryable.
• The expression can simply be a constant expression of the object itself or a more complex tree of a composed set of query operators and operands. The query provider's IQueryProvider.Execute() or IQueryProvider.CreateQuery() methods are called with an Expression passed to it, and then either a query result or another IQueryable is returned, respectively.
• IQueryable allows for out-of memory things like a remote data source, such as a database or web service.
• IQueryable is a very powerful feature that enables a variety of interesting deferred execution scenarios (like paging and composition based queries).

IEnumerable

• IEnumerable doesn’t have the concept of moving between items, it is a forward only collection. It’s very minimalistic; something that most any data source can provide. Using only this minimal functionality, LINQ can provide all of these great operators.
• IEnumerable is great for working with sequences that are iterated in-memory.
• IEnumerable just has a GetEnumerator() method that returns an Enumerator for which you can call its MoveNext() method to iterate through a sequence of T