Go to the API documentation and look for Map. When you open that, it gives brief details of what a Map is; you will find both the classes you quoted are Maps.
Read the descriptions of both classes. Particularly look for handling of null values, whether any methods are synchronized, whether there are any methods returning Iterators or Enumerations, and when the classes were introduced.
Most people doing modern Java programming use HashMap and don't use Hashtable, although this is not a "rule" or "regulation."
Additional: You will find HashMap and Hashtable underlined in blue when you find Map; those are links which will lead you where you want to go. [/edit] [ July 13, 2007: Message edited by: Campbell Ritchie ]
Hathtable is a legacy collection class, i.e., a legacy class from before Java version 1.2, when the Collections framework was added to Java. HashMap is more or less the replacement for Hashtable since Java 1.2.
In new programs, you should use HashMap and not Hashtable.