OK, the basic thing that all search engines cover. If you have text on your site, a search engine spider will read it and often index you. Text pages are returned in search results if the search keywords match their content and ranked according to the usual SEO factors including linking, keyword density, commitment etc.
Google Image Search "analyzes the text on the page adjacent to the image, the image caption and dozens of other factors to determine the image content." (image search FAQs). Some of these other factors may well include alt tags. So ensure your adjacent text, image caption and alt tag are all keyword relevant and targeted to the keywords you wish to rank for. If you are a news site and want your image to appear in Google News, ensure it is inline.
There are a number of things you can do to optimise videos. Especially important now video results are being returned in the standard Google search.
It was announced back in July that Google will index flash (SWF files). The original Google blog post has some more information on exactly how it works.
|Normal 'text and images' PDFs|
Google will search and index PDFs much as it does HTML. There's lots you can do to optimise your PDFs for search engines.
|PDFs of scanned docs|
Google have started indexing PDFs of scanned docs by using text recognition software (OCR) to 'read' the text. Announced at the end of October it's definitely worthwhile getting any PDFs of scanned docs online if you haven't already. Forgetting Google they may well be good content for your users anyway that will encourage linking.
|The previous 6 results summarize the main content types available online that can be indexed however also included in 'Universal Search' are a number of other types of content such as Google News and Google Product Search (formerly Froogle) that I haven't mentioned above as the on page optimisation remains the same - the results themselves are text - however it's essential you go through the necessary submission processes to get on these 'vertical search' engines (search engines providing something specific - the opposite of universal search really).|
To list some vertical search engines:
- News aggregators - Google News has submission guidelines. Also make sure images are inline as mentioned earlier. Don't forget other news aggregators such as NewsNow.
- Product search - Google Product Search as mentioned above, there are also many other search engines and sites that will list products.
- Business searches - Google Maps: Local businesses can add their details so they come up in searches. Yell.com: One of many traditional online business directories
- Blog searches - Get a blog and appear on a whole of host of search engines including Technorati and Google Blog search. This will also tap into the world of social bookmarking - which any page can do but blogging started it.
- Google has loads more including Google Book Search. Plus even more really specific searches but I think they are mostly done via various partners so you probably can't get on these.
- Mobile - And then last but not least it's worth considering search optimisation for mobile. With increased mobile internet usage search engines are going to have to start providing mobile friendly results.
- and many more...
Is structured data the future of search? (Mar 2010)