Google has a search engine. We're all familiar with it. It's at google.com. Within Google search, Google also has a lot of hidden search engines and hacks, such as converting currency, finding local weather forecasts, movie times, and finding stock quotes.
Search engines that search specific sub-groups of the web are known as vertical search engines. Google also calls them "specialized search." Google has quite a few of these specialized search engines. Many of these vertical search engines are deeply integrated into the main Google search engine — to the point that they really don't look any different from a regular Google search and can only be seen when you adjust your search settings. However, some of Google's search engines are separate search engines with their own URL. You might sometimes see a suggestion to try searching for those results in the main search engine, but when you're searching for a specific subject matter, it just saves time to go directly to the source.
If you search for academic research at all (including high school papers), you need to know about Google Scholar. Google Scholar is a vertical search engine dedicated to finding scholarly research.
It will not always give you access to those papers (plenty of research is hidden behind paywalls) but it will give you access to any open access publications and a direction to start searching. Academic library databases are often difficult to search. Find research on Google Scholar and then switch back to your library database to see if they have that particular document available.
Google Scholar ranks pages by taking into account the source (some journals are more authoritative than others) and the number of times the research has been cited (the citation rank). Some researchers and some studies are more authoritative than others, and citation count (how many times a particular paper is cited by other papers) is a widely used method of measuring that authority. It's also the method that was used as the foundation for Google's PageRank.
Google Scholar can also send you alerts when new scholarly research is published on topics of interest.
Google Patents is one of the more hidden vertical search engines. It's no longer as boldly branded as a separate search engine, although it does have a separate domain at patents.google.com.
Google Patent search can search through names, topic keywords, and other identifiers for patents around the world. You can view the patents, including the concept drawings. You can also use Google's patent search engine as part of a killer research portal by combining Google Patents and Google Scholar results.
Google used to have a vertical search engine that specialized completely in US government documents (Uncle Sam Search) but the service was discontinued in 2011.
Google Shopping (previously known as Froogle and Google Product Search) is Google's search engine for, well, shopping. You can use it for both casual browsing (shopping trends) or you can search for specific items and drill down into comparison shopping. You can filter searches by things such as vendor, price range, or local availability.
Results usually show both online and local places to purchase items. Information for local results is limited because it relies on stores to also list their inventory online. Thus, you're not likely to get as many results from smaller local merchants.
Google also had a related search engine that it discontinued, revived, and then killed again called Google Catalogs. It searched through print catalogs for shopping information.
Google Finance is a vertical search engine and portal dedicated to stock quotes and financial news. You can search for specific companies, view trends, or keep track of your personal portfolio.
Google News is similar to Google Finance in that it's a content portal as well as a search engine. When you go to the front page of Google News, it resembles a newspaper stitched together from a large number of different newspapers. However, Google News also contains information from blogs and other less traditional media sources.
You can customize the layout of Google News, search for specific news items. or set up Google Alerts to be notified of news events on topics of interest to you.
Google Trends (previously known as Google Zeitgeist) is a search engine for the search engine. Google Trends measures fluctuations and relative popularity of search terms over time. You can use it to measure general trends (lots of people are talking about Game of Thrones right now) or compare specific search terms over time. In the example image, we compared the relative popularity of 'tacos' and 'ice cream' over time.
Google also bundles Google Trends information for the year into the Google Zeitgeist report. Note that general trends represent changes in popularity, not a ranking of absolute search volume. Google indicates that the most popular search terms don't actually change much over time, so the trend data sorts out the background noise in order to find search phrases that are different.
Google experimented with a measurement of Google trends to find the spread of the flu, called Google Flu Trends. The project was started in 2008 and did quite well until 2013 when it missed the peak of the flu season by a large margin.
Google Flights is a search engine for flight results. You can use it to search and comparison shop between most airlines (some airlines, like Southwest, opt not to participate in results) and filter your searches by airline, price, flight duration, the number of stops, and time of departure or arrival. If this sounds a lot like the sort of thing you can already get on many travel search engines, that's because Google purchased ITA in order to make Google Flights, and that's still the same search engine that powers many of those travel sites today.
Google Books is a search engine for finding information in print books and a place to find your personal e-book library for any e-books you have uploaded or purchased through your library in Google Play Books. You can also easily find free e-books through Google Books.
Google Videos used to be a video uploading service that Google created as a competitor to YouTube. Eventually, Google gave up on the idea of building a full video streaming service from scratch and bought YouTube. They folded the video streaming features from Google Videos into YouTube and relaunched Google Videos as a video search engine.
Google Videos is actually a pretty amazing video search engine. You can find results from YouTube, of course, but you can also find results from Vimeo, Vine, and multiple other streaming video services.
When all else fails, make your own vertical search engine. Google Custom Search Engine allows you to make your own specialized vertical searches.
Google Custom Search Engine results display inline ads, just like standard Google search results.