Threading is a way of looking at the articles in a news group in terms of the specific subject that the article deals with. All articles that are submitted to a news group have a subject. A thread uses the subject to group articles as apposed to a general topic. You will find that in may newsgroups that there will be a number of articles that people will write that are about the same subject. The threads will die out mainly because of boredom related to the topic.
News servers are computers that collect information from around the world from other news servers.
The news administrators keep in touch with each other so that the usenet does not get out of control. Some newsgroups are actually run by and for news administrators so that they can communicate with each other easily. Internal Internal news servers are servers that contain information based on an organisation. External External is the news server which is connected to the usenet. It is a far more common news server than the internal one.