IEnumerable vs IQueryable

Both these interfaces are for .NET collections, 

The first important point to remember is “IQueryable” interface inherits from “IEnumerable”, so whatever “IEnumerable” can do, “IQueryable” can also do.

There are many differences but let us discuss about the one big difference which makes the biggest difference. “IQueryable” interface is useful when your collection is loaded using LINQ or Entity framework and you want to apply filter on the collection.
Consider the below simple code which uses “IEnumerable” with entity framework. It’s using a “where” filter to get records whose “EmpId” is “2”.

IEnumerable<Employee> emp = ent.Employees;

IEnumerable<Employee> temp = emp.Where(x => x.Empid == 2).ToList<Employee>();

This where filter is executed on the client side where the “IEnumerable” code is. 

In other words, 

all the data is fetched from the

 database and then at the client it scans

 and gets the record with “EmpId” is “2”.

But now see the below code we have changed “IEnumerable” to “IQueryable”.

IQueryable<Employee> emp = ent.Employees;

IEnumerable<Employee> temp = emp.Where(x => x.Empid == 2).ToList<Employee>();

In this case, the filter is applied on the database using the “SQL” 
query.  So the client sends a request and on the server side, a select query is fired on

 the database

 and only necessary data is returned.

So the difference between “IQueryable” and “IEnumerable” is about where the filter logic is executed. One executes on the client side and the other executes on the database.
So if you are working with only in-memory data collection “IEnumerable” is a good choice but if you want to query data collection which is connected with database, “IQueryable” is a better choice as it reduces network traffic and uses the power of SQL language.
Below is a nice FB video which demonstrates this blog in a more visual and practical manner.