Lazy, Eager, and Explicit Loading of Related Data http://www.asp.net/mvc/overview/getting-started/getting-started-with-ef-using-mvc/reading-related-data-with-the-entity-framework-in-an-asp-net-mvc-application

EagerLoading.cs

//Eager loading. When the entity is read, related data is retrieved along with it. 
//This typically results in a single join query that retrieves all of the data that's needed. 
//You specify eager loading by using the Include method.


//Query: All department rows and all related rows in one query
var departments = db.Departments.Include(d => d.Courses);
foreach (var department in departments)
{
    foreach (var course in department.Courses)
    {
        courseList.Add(department.Name + course.Title);
    }
}

ExplicitLoading.cs

//Explicit loading. This is similar to lazy loading, except that you explicitly 
//retrieve the related data in code; it doesn't happen automatically 
//when you access a navigation property. You load related data manually by getting the object state 
//manager entry for an entity and calling the Collection.Load method for collections 
//or the Reference.Load method for properties that hold a single entity. 
//(In the following example, if you wanted to load the Administrator navigation property, 
//you'd replace Collection(x => x.Courses) with Reference(x => x.Administrator).)  
//Typically you'd use explicit loading only when you've turned lazy loading off.

var departments = db.Departments.Include(d => d.Courses);
foreach (var department in departments) //Query: all Department rows
{
    db.Entry(department).Collection(x => x.Courses).Load(); //Query: Course rows related to Department 'department'
    foreach (var course in department.Courses)
    {
        courseList.Add(department.Name + course.Title);
    }
}

// Lazy loading
//viewModel.Enrollments = viewModel.Courses.Where(
//    x => x.CourseID == courseID).Single().Enrollments;
// Explicit loading
var selectedCourse = viewModel.Courses.Where(x => x.CourseID == courseID).Single();
db.Entry(selectedCourse).Collection(x => x.Enrollments).Load();
foreach (Enrollment enrollment in selectedCourse.Enrollments)
{
    db.Entry(enrollment).Reference(x => x.Student).Load();
    //Notice that you use the Collection method to load a collection property, but for a property that holds just one entity, you use the Reference method. 
}

LazyLoading.cs

//Lazy loading. When the entity is first read, related data isn't retrieved. 
//However, the first time you attempt to access a navigation property, 
//the data required for that navigation property is automatically retrieved. 
//This results in multiple queries sent to the database — one for the entity itself 
//and one each time that related data for the entity must be retrieved. 
//The DbContext class enables lazy loading by default.

var departments = db.Departments;
foreach (var department in departments) //Query: all Department rows
{
    foreach (var course in department.Courses) //Query: Course rows related to Department 'department'
    {
        courseList.Add(department.Name + course.Title);
    }
}