What are Navigation Properties in Entity Framework for?

I see in my EF diagram alot of these navigation properties but not sure what they are really for. Like I see in lots of my tables I have aspnet_Users properties.

What are these for? Do they help for joins? or what?

Error 2
Error 3007: Problem in Mapping Fragments starting at lines 1201, 1423: 
Non-Primary-Key column(s) [Field2] are being mapped in both fragments 
to different conceptual side properties - data inconsistency is possible 
because the corresponding conceptual side properties can be independently 
modified.

A navigation property allows you to navigate from one entity to a "connected" entity.

E.g. if your user is connected to a role, you can use the "Role" navigation to read and inspect the role associated with the user.

EDIT:

If you want to load the user with LINQ-to-Entities, and also look at its "Role" navigation property, you have to explicitly include the "Role" entity in your LINQ query - EF does NOT load those navigation properties automatically for you.

  // load user no. 4 from database
   User myUser = from u in Users.Include("Role")
                 where u.ID = 4
                 select u;

   // look at the role the user has
   string roleName = myUser.Role.Name;

OR:

   // load user no. 4 from database
   User myUser = from u in Users
                 where u.ID = 4
                 select u;

   // check to see if RoleReference is loaded, and if not, load it
   if(!myUser.RoleReference.IsLoaded)
   {
      myUser.RoleReference.Load();
      // now, the myUser.Role navigation property should be loaded and available
   }

   // look at the role the user has
   string roleName = myUser.Role.Name;

It's basically a programmatic equivalent to a foreign key relationship in a database - a connection between two objects. It basically "hides" or resolves a join between two tables (or two entities, in EF speak).

Marc