Headers and Overviews
Each news article contains a complete set of header lines, but in common use the term "headers" is also used when referring to the News Overview database. The overview is a list of the most frequently used headers, and additional information such as article sizes, typically retrieved by the client software using the NNTP XOVER command. Overviews make reading a newsgroup faster for both the client and server by eliminating the need to open each individual article to present them in list form.
If non-overview headers are required, such as for when using a kill file, it may still be necessary to use the slower method of reading all the complete article headers. Many clients are unable to do this, and limit filtering to what is available in the summaries.
Read more about this topic: News Server Operation