1. "Search engines search the entire Web!"
When you use a search engine, you're searching a proprietary database that contains a lot of information about the Web, including copies of a certain number of Web pages. Most search engine databases, called indexes, do not even come close to representing the entire Web. In fact, the vast majority of the Web doesn't even show up in a cursory search - see The Invisible Web : A Beginners Guide to the Web You Don't See for more information on this fascinating treasure trove of data.
Duplicate pages and spam that clutter a search engine's index can make searching even more difficult. Add social networking updates, Twitter statuses, Facebook information, and Google+ input into those same search results, and you've got quite a mess.
Why do search engines use an index instead of searching the Web directly? Those millions of documents on the Web are located on hundreds of millions of different "host" computers all over the world. It would take days, maybe even weeks, for even the most powerful computer to simply find and contact all host computers, let alone to search each one of them individually in real time and return timely results.
Searching the Web directly would be like going door-to-door in Los Angeles looking for a friend, rather than looking up her address online. Instead, what search engines do is send out "spider" programs to roam the Web and download pages into their database. It's much faster to search a single database than the whole Web.
Gradually, these databases of Web pages begin to look like "mirrors" of the Web, though they are very incomplete, and because of the time it takes to compile indexes, often out of date. This is the reason you occasionally get "404- Not Found" messages when you click on a link in a search result list; the Web page was removed before the spider could make a return visit and "refresh" the index.
So, the next time you use a search engine, remember you are not searching the Web itself, but rather a proprietary database. Which brings us to the next myth.
2. "All search engines are exactly the same!"
Most search engines have a similar appearance. They all have a search form of some type, they all have links to different category topics, and they all return lists of results (SERPs) when you enter a search query.
However, that's where the resemblance ends. The major search engine services all have distinct "personalities," and will give you quite different results for nearly every search you attempt. It makes sense to think of them as people. For instance, in your own life, you probably have friends or acquaintances who have expertise in certain areas. You might ask your athletic friend Connie for advice on running shoes, for example, whereas you'd ask your geeky friend John for guidance on computer issues.
Next time you're using a search engine, try to figure out its "personality", with an eye toward understanding the kinds of questions it answers best. Read How to Pick the Right Search Engine for Your Needs for a more in-depth exploration of this topic, in addition to suggested search engines for a wide variety of search queries.
3. "Finding stuff online is easy!"
There's no arguing that Web search has advanced by vast leaps and bounds in the last few years. A search for something today will give you completely different results than a year ago; even from just one week ago. Information on the Web changes dramatically day to day because there is such an overwhelming glut of it. However, that does NOT mean that finding this information is an easy task.
Search engines can trick you into feeling a false sense of security. It's easy to believe that if you found a page once using a search engine, you needn't bother to bookmark it or mark it as a favorite. Why, just re-enter your search query, and you'll have no problem finding the site again. Sorry, it doesn't work that way. In fact, you may not get the same results if you repeat a search within the hour, let alone days or weeks later. There are several reasons for this.
First, the Web is in constant flux. Hundreds of thousands of new pages are published to the Web every day, and thousands more are moved to new "addresses," or are removed entirely. This means that the "relevance" of a particular document for a specific search query also changes constantly, as it is compared to other documents added to or removed from the index.
Second, Web page authors can also manipulate relevance rankings, to a certain extent, so pages that once ranked highly can be bumped by pages that have been tweaked to achieve higher relevance. This is a contact sport for some Web authors, and they spend hours and hours trying to outwit the indexes, occasionally with remarkable—but usually transitory—success (see How Does Search Engine Optimization Work? for an expanded discussion on this subject).
Third, the sheer volume of information available is distracting. Finding citeworthy sources that are worthy of that citation is bewildering, especially if you're not sure what makes a source reliable. Learning how to evaluate a Web source for credibility is paramount when using any online content as a source. For more information on finding good sources online, read Where is the best place to start research on the Web?, The Best Reference Sites, or How to Search More of the Web.