Hummingbird is the code name for the new release of the Google algorithm announced in September 2013. While the Panda and Penguin releases were updates to the Google algorithm, Hummingbird is a complete new release. The major change in the algorithm is the focus on semantic search.
While keyword search remains important, there is an increased emphasis on sematic search, where the information is retrieved based on user intent and the contextual meaning of the search words entered.
The Hummingbird update extends Googles use of Knowledge Graph. Knowledge Graph leverages 'form based queries' and 'natural language queries' to answer questions and provide context based results. In other words, the user can ask a question and this will be interpreted as part of the search process (see this article by Barbara Starr for more information).
SEO (some advice)
The Hummingbird update should benefit sites that provide quality content, sites that make good use of markup and sites that conform to web standards. As the Google algorithm moves more and more towards verification of data sources (trust, recognition, brand awareness etc.) it becomes more important to use attributes like rel = author and rel = publisher. Finally, keep the customer (searcher) in mind when designing the content for your site.
"Google Hummingbird is mainly about Semantic Search"