How to make an Internet Search Engine?
Easiest way is to just write an application that visits web pages either at random, following clues or in an ordered pattern, or using a mix of these.
After visiting a page, it's contents can be downloaded for analysis & categorization.
This process needs to be repeated after a given time period, to check if web page content changed, and needs to be re-categorized.
This takes resources such as computer memory, cpu-cycles, the Internet bandwidth, is race against time... but even without large amount of resources, parts of the Internet can be visited still, and it's cheaper if we know what we are looking for.
Then downloaded & categorized data can be presented to users, who write 'queries' in search field and get results of the search in response - picked from data that has been prepared for them in previous steps.
Sorting Server, Parallel or not, can be useful for preparing large amounts of data for categorization, presentation, perhaps for more.
Various the Internet Search Engines can serve different purposes, depending on which, methods of search, categorization & presentation can vary.
To write a Search Engine that seeks Terrorist Threats in the Internet, application writer needs to understand Terrorism - it's language, thinking patterns, methods of communication & symbolism, perhaps more. He or She needs to be prepared to handle that properly as well. This is what i lack at, at least for now.
Knowledge of Cryptography helps too.