GNU Smalltalk is an implementation which is almost Smalltalk-80 conform ([tm] ParkPlace Systems) as described in Adele Goldberg and David Robson's book 'Smalltalk-80: the Language and its Implementations', which will be referred to as the 'Blue Book'.
Smalltalk is an object oriented programming language, meaning that the programmer not only has to keep the data in his head but the operations which can be applied to this object. The ability of an object of keeping not only track of the data but also to those operations that are connected to itself are inseparable glued together. The amount of actions that can be performed by the object is precisely defined by the amount of operations of this special object (called methods in Smalltalk). You can't 'see' the contents of an object from outside the object. Seen from the outside an object is a black box which has some particular operations and states, but that's about all you know.
In Smalltalk everything is an object. This includes variables, executable procedures (methods) and stack frames (context of invoked methods or context of a block). Every object is an instance of a class. You can see a class as a datatype and its incorporated functions. An instance is a particular variable of this datatype.
If you want to perform an operation on one of these objects you have to send it a message and the object reacts by performing the correspondent action. To print something, e.g., you could send the message like this:
someObject print
The message you send is indeed the name of a method (procedure) which should be carried out. If you send a message to an object Smalltalk tries to find out the method that has been defined for this kind of object. First it starts looking at the objects class. If there is no such class it bothers the parent class, in the grandfather class etc.. At the top of this class-construction there is a class called Object which has no ancestors. If Smalltalk couldn't find a method until then it produces an error code.
For a complete explanation of Smalltalk there is a tutorial which is provided some extra example programs.
Additionally the X Smalltalk interfaces 'stix (gst_stix) as well as the TCP interface (gst_tcp) are included.