1) CType is capable of a *cast* or a *conversion*. DirectCast can only
*cast*

By "conversion" I mean converting one datatype to another (e.g. string to
integer, decimal to integer, object to string etc).

By "cast" I mean changing one type of object into another type that is
related to it by one of the following rules:

a) The type you're converting the object to must be the same type
e.g.
--------------
Dim a as String = "hello"
Dim b as Object = a
Dim c as string = directcast(b, String)
--------------

Variable b is an object that holds a string, so you can cast it to a string.

b) If converting to an interface, the type you're converting must implement
the interface
e.g.
---------------------
Dim a as New MyInterfaceObejct
Dim b as IInterface = directcast(a, IInterface)
----------------------

c) If converting to a derived type, the runtime type of the object must be
the derived type or one of it's own derived types :S
e.g.
----------------------
Dim a as Base = New Derived
Dim b as Derived = directcast(a, Derived)
----------------------

2) Use directcast whenever a "type relationship" exists - it is slightly
faster (in some cases anyway), but forces you to be more aware of
conversions that are going on.

3) Use Ctype when a type relationship doesn't exist, but a value
relationship does (e.g. converting the string "123" to the integer 123)