Often it is necessary to retrieve an entity or a specific column of an entity with an organization service execute request (retrieve request). With this Request you’ll retrieve the actual database value of that entity record. But you also have to know the actual Id of that record.
With the ColumnSet you can definde which columns you want to retrieve. By setting the ColumnSet to true all columns will be returned.
Contact contact = (Contact)organizationservice.Retrieve(Contact.EntityLogicalName, contactId, new ColumnSet(true));
Learn more about this on MSDN.