Lazy Loading vs Eager Loading

Eager Loading: Eager Loading helps you to load all your needed entities at once. i.e. related objects (child objects) are loaded automatically with its parent object.

When to use:

1. Use Eager Loading when the relations are not too much. Thus, Eager Loading is a good practice to reduce further queries on the Server.
2. Use Eager Loading when you are sure that you will be using related entities with the main entity everywhere.

Lazy Loading: In case of lazy loading, related objects (child objects) are not loaded automatically with its parent object until they are requested. By default LINQ supports lazy loading.

When to use:

1. Use Lazy Loading when you are using one-to-many collections.
2. Use Lazy Loading when you are sure that you are not using related entities instantly.

NOTE: Entity Framework supports three ways to load related data - eager loading, lazy loading and explicit loading.

Consider the below situation

public class Person{
    public String Name{get; set;}
    public String Email {get; set;}
    public virtual Employer employer {get; set;}
}

public List<EF.Person> GetPerson(){
    using(EF.DbEntities db = new EF.DbEntities()){
       return db.Person.ToList();
    }
}

Now after this method is called, you cannot lazy load the Employer entity anymore. Why? because the db object is disposed. So you have to do Person.Include(x=> x.employer) to force that to be loaded.

Lazy loading will produce several SQL calls while Eager loading may load data with one "more heavy" call (with joins/subqueries).

For example, If there is a high ping between your web and sql servers you would go with Eager loading instead of loading related items 1-by-1 with lazy Loading.

I think it is good to categorize relations like this

When to use eager loading

1. In "one side" of one-to-many relations that you sure are used every where with main entity. like User property of an Article. Category property of a Product.
2. Generally When relations are not too much and eager loading will be good practice to reduce further queries on server.

When to use lazy loading

1. Almost on every "collection side" of one-to-many relations. like Articles of User or Products of a Category
2. You exactly know that you will not need a property instantly.

Note: like Transcendent said there may be disposal problem with lazy loading.