When you send a message to a class, you invoke a class method. When you send a message to an instance of a class, you invoke an instance method.
Most class methods create new objects of that class.
There are other uses of class methods, but until you understand them, just stick to using class methods for creating instances. Note that a class is an object. But in Smalltalk, everything is an object. So of course, classes will be objects, too. Smalltalk does not need constructors, because class methods can act like constructors by creating a new object and then initializing it.