Wikipedia: “ Search engine optimization ( SEO ) is the process of improving the visibility of a website or a web page in search engines via the "natural" or un-paid ("organic" or "algorithmic") search results.
SEO = Optimizing webpages, images, videos, locations, products, articles, etc. to make them show up higher on search engine results, without paying for it (unless you are hiring someone to do it for you).
SEO ≠ SEM
SEM = Search Engine Marketing
Paying for results to show up on page 1 results of search engines (advertisements)
“ Crawlers” to scan the internet and index all of the web pages
Google Webmaster Tools includes reports on crawl errors (when the page fails to be indexed) www.google.com / webmaster s/ tools
Use Webmaster Tools to determine the reason for crawl errors
Research how to fix the problem (in most cases, either the URL has changed and it needs a redirect to the new URL, or the page has been removed altogether [redirect to home page or similar page in this case])
How to redirect: http://www.webweaver.nu/html-tips/web-redirection.shtml