Website Magazine has listed some of the most important changes related to Google SEO below by generalized category, but do review the full list which focuses on the search experience for consumers as much as anything of note for SEO professionals. In line with previous search quality change reports, Google has assigned codenames for each change which does make it easy to keep track of the impact of the changes in the future.
For the first time in my recent memory, Google noted that it increased the size of its base index by 15%. The base search index is Google's main index for serving search results. Google also introduced a new "index tier". Google keeps its index in tier where "where different documents are indexed at different rates depending on how relevant they are likely to be to users."
And now, on with the Google SEO changes to know!
LOCAL RELATED: Google made some significant modifications related to language relevance and country identification for Web pages. The "Raquel" update takes into account language to help return relevant navigational results, while Sudoku improves the systems currently in place to detect when a website, subdomain or directory is relevant to a set of countries and does so down to the page level. The "ImpOrgMap2" change is likely the most important local related change in this set, which makes it more likely users will find a website from a specific country - e.g. mexico.cnn.com over cnn.com - in the search results pages.
SNIPPET RELATED: Several changes were also made to snippets in April 2012. Google updated its system for generating snippets with the project codename "DSS" to keep it consistent with other infrastructure improvements and is now more likely to show text from the beginning of a page in snippets when the text is particularly relevant - codename Solar.
FRESHNESS RELATED: Perhaps the most noteworthy and actionable of all changes released in this round are related to freshness. Google indicated that its "Citron" update enables the search engine to better identify fresh documents, but its "NoRot" update modified a classifier to ensure content that is identified as low-quality" will be excluded - even though its fresh.
SPELLING RELATED: Google released a set of changes related to spelling corrections. The "Potage" update internationalizes one of Google's algorithms to prevent bad spell corrections; the "Pita" update extends spelling corrections to more than 60 languages, and the "Spelling" update is a change which makes it more likely that queries get a spell correction even if it's longer than ten terms.