The usual meaning of "Smalltalk" is either the Squeak programming language or the historical programming environment that Squeak derives from. Note that the "t" in Smalltalk isn't capitalized.
Squeak is a descendent of Smalltalk-80. The core developers of Squeak (Squeak Central) in fact include many of the core developers of Smalltalk-80.
Squeak's language and much of the class library is identical to that of Smalltalk-80: they both have objects, classes, single inheritance, blocks, garbage collections, collections, streams, model-view-controller, and many other bits.
ANSI Smalltalk is a recognized standard for Smalltalk. While Squeak is Smalltalk in spirit, it is not currently (2003) compliant with the ANSI Smalltalk standard. There is temporarily an add-on on SqueakMap which tries to move Squeak towards ANSI compatibility.
The Smalltalk object contains all global variables(including itself), most of which are references to classes. Additionally, the Smalltalk object responds to system-wide requests like