In an effort to be more secure, Google no longer tells websites what keywords were responsible for directing traffic to the site. The only exception is paid-for search results, which still come with keyword data.

The change to the way keyword data is handled is ostensibly due to Google wanting to keep us all safer on the web and is a reaction to the recent NSA/PRISM surveillance revelations.

The main change is that, as long as you are signed in to Google, future searches will be made using a secure SSL. That is, search data will be encrypted so that no eavesdropper can discover what you are searching for, i.e. the keywords you used, and they can't read your search results page.  This all sounds like a good idea and you can even go directly to https://www.google.com if you want use encrypted search without being signed in. 

The fact that SSL is used for the connection doesn't mean that Google has to alter what is sent to a website when you eventually click on a link. However, for the same reasons of security, Google has decided to only provide the information that the user came from a Google search and not the search query that resulted in the link being displayed. 

The only data that will be available in the future is an aggregated list of the top 1000 search queries for the most recent 30-day period. To get this you have to use Google Webmaster Tools. 

What this means is that a long standing source of information on what queries were driving traffic to a site will no longer be available, making some forms of SEO and web design not possible. In particular, if you design a web page that reacts to the search query by showing additional relevant items this will no longer work. Similarly, sites that used keyword optimization to attract traffic, like the Huffington Post, are going to have to find a new way to discover what keywords are important.

Google may be using the switch to SSL as some sort of reason for cutting the flow of data, but this is just a smoke screen.  There is no inherent technical reason to strip out query information from the referral link. Indeed if a user clicks on a paid-for, i.e. Adsense, link then the website still gets the query information.  Google justifies this as:

If you choose to click on an ad appearing on our search results page, your browser will continue to send the relevant query over the network to enable advertisers to measure the effectiveness of their campaigns and to improve the ads and offers they present to you.

So for organic, i.e. unpaid, links issues of privacy matter, but for paid for links they don't. This doesn't make much sense.

As you might guess the marketing community is upset by the change and it is even being referred to by the term "Data Apocalypse". 

It just emphasises the fact that Google is a de facto monopoly on search - Bing has 17% to Google's 70% - and can more or less do what it likes as long as it doesn't alienate the user. In this case making a case for better privacy while still selling the same data to advertisers will probably go unnoticed by the user, if not by the marketer.