Google and Yahoo often show different results when you search, but why? The simple answer is that each search engine uses a different method to guess what it is you want to see. They keep track of what other people looked for, to help decide what might be relevant to you, and sometimes they'll also put their own pages in the results, for advertising reasons.
Search engines have a lot of different "rules" to decide what order of results to show you. Some of these rules include reading the name of the website, the headings, words in the text, image names, information you can't see (in the behind-the-scenes coding) and also which website links are included. This set of rules is called the algorithm, and we can learn a little bit about these rules.
The first thing to understand is that Google and Yahoo are both big, wealthy sites who make money from advertising. They like putting their own results up the top, so that you click them and buy things. Fortunately Yahoo and Google still show other people's sites along with the paid ads, but the two sites still have some big differences in what comes up.
Each search engine has a different opinion on what matters the most. If we look closer here are the main differences from Google to Yahoo:
Google looks closely at a web page's text and also where it links to. It can tell a little better if the links make sense, and manages to filter out a lot of sites that have pages full of nonsense, or fake and broken links. (Pretend links, and pages full of nonsense words, are how some website owners try to get themselves on top of the search results.)
If a website has Lots and lots of links to different sites, they need to be relevant - if they are links to completely different subjects, Google won't rank it as highly as a page full of relevant links. So if you're looking for Ice Cream and a certain Ice Cream website has lots of links to Cadillacs and Knitting, you probably won't see it near the top!
Yahoo will often include websites filled and overflowing with one particular word - again which the website's owner has repeated over and over to try to get themselves up to the top of the search results. Google realises that it's probably not the best article if it's just full of the same few words, and Google will sometimes remove that site from the results shown to you.
Yahoo isn't quite so good at stopping "clone" sites appearing more than once. Ten identical websites all saying the same exact text isn't very useful to you, and Google tends to keep these out a bit better.
Overall, Google tends to supply general information results, and Yahoo's results tend to be a little towards the shopping side of things - even if those results aren't paid advertising for Yahoo. If you're shopping, that might make Yahoo a great choice for you.
So there you have it. There are technical scientific reasons of course, but the search engines guard these fiercely from our eyes. Which one you use comes down to personal preference. I prefer Google as it feels a little less biased, but a true shopaholic will love Yahoo. So enjoy whichever one suits you best.