They're the search engine spiders, and they are in control of probably the largest section of your traffic. You need to please these spiders if you want your site to be successful. Here's how.
Make Your Structure Clear.
Resist the temptation to lay your page out in non-standard ways: you want it to be very clear to the search engine where the navigation is, where the content is, and where the headings are. As a rule, put navigation first in your page. Always use the heading tags (h1, h2, etc.) for headings and sub-headings.
Avoid using generic span and div tags and only making things clear to the user through CSS font sizes: instead, use every 'semantic' HTML tag that applies to your content. If you're quoting someone, use the blockquote tag; if you're posting program code, use the code tag. Search engines love this.
Keep Keywords Consistent.
It's not usually worth deliberately saturating your content with keywords in hope of a higher search ranking – the engines have pretty much wised up to this tactic – but do make sure that your keywords appear consistently when they occur naturally. For example, for these articles, I have stuck with 'website' throughout, as suddenly writing 'web site' instead would bring down my rankings.
Use Meta Tags.
Yes, meta tags are out of fashion, and search engines pay no attention to them any more when it comes to ranking your site, but they're still important in one way: the meta description tag is still often used to decide what text search engines' users see when they find your site in their results! This can be just as important as the ranking itself – write something here that will look useful to the searcher, and you're more likely to get them to click-through. Don't forget that, while search engines are just machines and algorithms, the end result of it all does involve a human decision: to click, or not to click?
Avoid Splash Pages.
You might think it's a great idea to have a 'splash' page displaying a full-page version of your logo (or an ad) to every user who arrives at your site, but search engines really hate that. Using this trick will get you ranked far lower than you would usually be, so you should avoid it – it's annoying to visitors anyway.
Include Alt Tags.
Any time you use a graphic, include alt text for it – especially if there is text in the graphic. Remember that, as far as search engines are concerned, all your graphics might as well just be big black boxes. Test by removing all your graphics and seeing if your content remains relatively intact. If it doesn't, then you'll be turning search engines away.
Finally, Write Great Content.
The key with modern search engines (and, at the same time, the thing you have least control over) is how many people decide to link to your page from their page. How can you make more people link to you? Make your content useful. Make it something they'll want to quote on their blogs. Content is more King than it's ever been, and the best way to design for search engines is to make your content really stand out.