A constructor is a defined method on a type which takes a specified number of parameters and is used to create and initialize an object.
An object initializer is code that runs on an object after a constructor and can be used to succinctly set any number of fields on the object to specified values. The setting of these fields occurs after the constructor is called.
You would use a constructor without the help of an object initializer if the constructor sufficiently set the initial state of the object. An object initializer however must be used in conjunction with a constructor. The syntax requires the explicit or implicit use (VB.Net and C#) of a constructor to create the initial object. You would use an object initializer when the constructor does not sufficiently initialize the object to your use and a few simple field and/or property sets would.
class_name() -> this makes a call to the constructor which is preceded by the 'new' keyword to create an object.
new class_name() -> this complete line will create a new object by calling the constructor of the class.