Getting to know Smalltalk
Some interesting extracts from well-written articles (see References):
The biggest difference is probably the fact that a Smalltalk implementation is not just a programming language but an Image-Based system.
... the unit of compilation in Smalltalk is a single method. In Smalltalk, the programmer edits a method. After finishing, the programmer clicks a button and the method is re-compiled. The compilation is instantaneous, much faster than C++ or Java. Furthermore, there is no separate link or load phase.
With Java/C++, there is a clear distinction between the "compilation" phase and the "run-time" . Smalltalk is different in this respect; the Smalltalk system is running at all times and all classes (about > 1500 of them) are present and loaded all the time. When new a class is created, it is added incrementally to the running system.