Jim hits all the right notes. I would add a couple other notions...
There is no
main. Rather than writing programs, you construct objects. Rather than running programs, you test objects. A Smalltalk environment will have several browsers up at a time. Some for browsing, others for editing. Another typical scenario is to have several *workspaces* up at the same time. A little code here, a few tests there.
This ties into my other notion, that Smalltalk is essentially *testable*. Before the onslaught of XP, Smalltalk culture was even then based on writing a little and testing a little.
Plus common practice (trivially achieved in the Smalltalk environment) is to keep this "image" of multiple windows and multiple, incremental tasks flowing between sessions. Smalltalk has trivial persistence that supports the development environment greatly.