These are like the Library's selected web sites, but they contain far more sites and are usually maintained by a group of librarians and other subject specialists. Try the Mechanical Engineering or Manufacturing Engineering sections of EEVL . Search Engines allow you to 'search the whole Internet'. They automatically find and index web pages, and add them to their database. Most rank pages according to how often your search terms appear, how high up the page they are, and how close they are together.
Many search engines have a subject tree as well.
Often you can browse down to a certain level and then search within that subject area. Try out a few different search engines and see how they compare. Have a good look at their help pages to see what advanced search features they offer. Most people then tend to stick to one or two search engines, but it is a good idea to try out others once in a while. Some people prefer to use metasearch engines like DogPile or MetaCrawler - these search many search engines at once, but give you less control over how you specify your search.