Google maintains control of search, You Tube, the Android operating system and apps, while other offshoots of the former company including Calico, Google Ventures, Google Capital, Google X, and Nest Labs are, like Google, Alphabet subsidiaries, each of which has its own CEO.
Alphabet was founded October 2, 2015 by Google co-founders Sergey Brin, who will serve as president of the new company, and Larry Page, who will serve as CEO. Google's former Product Chief, Sundar Pichai, is now CEO of the subsidiary. Google shares were transferred to Alphabet stock on a share-for-share basis. The new company trades under Google's former ticker symbols, GOOG and GOOGL.
According to Larry Page's announcement, they chose the company's name because the alphabet is "a collection of letters that represent language, one of humanity's one of humanity’s most important innovations, and is the core of how we index with Google search." Page also said they liked the fact that the name breaks down into Alpha-bet. In a financial context, Alpha means investment return above benchmark.
Matt Schaefer explains Google's reorganization into Alphabet:
Fun fact: The company's announcement release about Alphabet included an Easter Egg. To see it, click the period after "drone delivery effort."