A subject tree is simply a list categorized by topic that has been gathered by a group or individual. One thing to remember about subject trees is that since they are gathered by people, they may be biased, therefore, knowing why the list was developed may help you know how helpful a particular list will be. There is no need for an individual to know specifically what they are looking for using a subject tree, as one can browse, almost like browsing chapters in a book. Another time subject trees can be very useful is finding a particular company homepage. A search on a search engine will give hundreds or thousands of hits on the words Reebok or Netscape, while a subject tree will have the company homepage listed under topics like Business or Companies.
Search engines are best at finding a very specific answer.
If you want to combine two or three terms, you probably want a search engine. Search engines consist of a robot program that gathers information from the Web, an index made from this information, a search tool that you use to ask for the information and a result page. To have the most luck using a search engine, it is important to read the instructions. There will be different answers for different search engines. One last hint - try various search tools, and observe the differing results.