Google has updated its search algorithm for the first time since 2010. According to this post in the Bits Blog on the New York Times site, this is the most extensive overhaul of the algorithm since 2000.
Although Google is not revealing specific details of the new algorithm, code named "Hummingbird," the main thrust of the changes are geared toward understanding longer and more complex queries. To that end, Google is employing its Knowledge graph, which is a map of semantic relationships.
One of the drivers for understanding longer queries is that more people are speaking queries into their phones using natural language, which is by its nature more complex than a simple keyword query typed into a search box.
For more information, see:
Search Engine Land FAQ on Hummingbird:
Google "15th anniversary" blog post (includes a great "Google Search Timeline" graphic):
Google post on the Knowledge graph (2012):