Casting is an explicit type conversion, specified in the code and subject to very few rules at compile time. Casts can be unsafe; they can fail at run-time or lose information.
Implicit conversion is a type conversion or a primitive data conversion performed by the compiler to comply with data promotion rules or to match the signature of a method. In Java, only safe implicit conversions are performed: upcasts and promotion.\
Also I would suggest reading about C++ implicit coversion: http://blogs.msdn.com/b/oldnewthing/archive/2006/05/24/605974.aspx
