PUBLIC MARKS from webgratuito with tag ranking

When you search the Net, you type in a certain keyword base on what you wanted to find. And then after you have done that, the search engine will give you pages containing a number of sites that contains the keyword you enter. The sites even appear according to the search engine ranking. Can you imagine how does a specific search engine can do that? It browses a million of sites to find the information you are looking for and gives you results in an instant!