Once upon a time - okay, about 1996 or so - there was a race for dominance of this new thing called the World Wide Web. On one side was Microsoft, having firmly staked its ground a few years before with the introduction of Windows 3.0. On the other side was a little startup called Netscape that had only one product - a web browser - that it had been beta testing for a couple of years for free but was now faced with trying to get people to actually pay for said browser. So one of the programmers there, a bright guy by the name of Brendan Eich, came up with a novel concept - a simplified scripting language that borrowed some of the syntax from the new Java language that Sun, just down the road in the Bay Area, had debuted a few months earlier, added in a few constructs that were a bit groundbreaking, and made it possible to alter the fields of the new HTML form controls that had just come out.
NoSQL. The 1990s saw the rise of the relational database, and traditional SQL databases played a big part in the way data was stored through the late 00s. However, one of the major side effects of AJAX was a fairly fundamental shift in the way that data was treated. Prior to then, most applications communicated with a SQL database through some kind of recordset abstraction layer, and the 90s especially were littered with three letter data access layer acronyms. However, one of the things that began to emerge once AJAX came into play was that data was seldom flat - it had structure and both clearly defined and not-so definitive joins of various kinds, and more often than not more closely resembled a document rather than simply a bag of property values. About this same time, data in motion really began to become more broadly important than data at rest, and for that reason the denormalization of data that was typically involved with such structures began to seem archaic when those structures already had clearly defined, and multi-tiered, layers.
NoSQL databases - a wide gamut of non-traditional databases including XML, JSON, name/value, columnar, graph and RDF stores - had been percolating in the background during much of the 00s. As system performance improved, these stores (which usually embedded not just single tables but whole DOMs) began to relied upon, first for document-centric information, then later for data-centric. AJAX gave them a much needed push, as being able to store complex data structures directly without having to reconstruct implicit joins reduced the overall data access burden considerably, and also moved much of the computing world to a more resource oriented (or RESTful) mode of operation.
Map/Reduce. Google's search capabilities are made possible primarily by creating massive indexes of the web. The technique for building these indexes in turn comes from a powerful application of a concept called map/reduce, in which incoming data (such as web pages) is mapped into large blocks, each block is processed for indexing, then the subsequent indexed content is reduced into a single indexed data set. By splitting the content apart then reintegrating, such a process can be transformed into multiple parallel processes, each running on its own real or virtual server space with dedicated processors.
HTML5, Android and Mobile. The HTML specification was, for all intents and purposes, frozen at version 4.0 in 1997. Several proposals for expanding outliers to the language - a graphical language (or two), CSS for layout, video and audio, new controls and so forth - had been building up in the background, but it was ultimately a decision to create a new version of the specification by outside groups that ended up providing the impetus to try to tie all of these pieces together. The resulting set of technologies was very sophisticated, including capabilities for managing both 2D and 3D graphics, sophisticated enough that it satisfied several key requirements for a graphical user interface ... and put the final nail in the coffin of third party binary plugins for the browser.
Google purchased the Android core set in 2005, and built on top of that for several years until the nascent smart phone market exploded with Apple's introduction of the iPhone in 2007. By the time the first Android phone was released in 2008, it's Java based operating system was firmly set. However, Java for Google is somewhat problematic - it does not control the technology, and the Android OS has a number of vulnerabilities that arise precisely because of the use of Java.