When you "google" something, you are using a search engine. Knowing a little bit about how search engines work, and understanding their limitations, can help you get the best out of them.
Search engines know about more sites than we human beings ever will. This is because they are collecting information constantly through automated processes called "spiders"—programs that "crawl" the Web. These spiders go from link to link and extract specific information, such as the URL, the title of the page, and the text. Depending on the type of search engine, they may extract other information, such as images, as well.
The spiders deposit the information they collect in databases, or indexes. When you do a search, the search engine scans its index, and, using a complicated formula, or algorithm, decides which pages in its index match your keywords. It then presents them to you in order of relevance—in other words, it puts the ones it thinks will most likely suit your needs at the top.
According to one measurement released in May 2009, Google is used for 65% of all Web searches, more than three times the market share of Yahoo!, its nearest competitor. Clearly most people think that Google brings the best results. But since each search engine has its own spiders and its own algorithms (which are guarded as valuable trade secrets!), each will return different results. If you are searching the Web, it might be a good idea to use more than one.
In this section we will mostly be discussing Web search engines—search engines that index Web pages. But you should also be aware that some engines only index certain types of material, so if you're looking for a particular type of material you might try them. Google and its competitors all have separate search engines for video, news, and blogs. Google even has a search engine for books it has scanned: Google Book Search. Only a small number of books are available to read in full this way, but you can search many others, so this might be a good way to find book materials that you can then get from the library. (Click the image for an example of Google Book Search results.)