The Open Web is a set of philosophies; these philosophies include:

Decentralization - Rather than controlled by one entity or centralized, the web is decentralized -- anyone can create a web site or web service. Browsers can work with millions of entities, rather than tying into one location. It's not the Google or Microsoft Web, but rather simply the web, an open system that anyone can plug into and create information at the end-points.

Transparency - An Open Web should have transparency at all levels. This includes being able to view the source of web pages; having human-readable network identifiers, such as URLs; and having clear network entry points, such as HTTP and REST exposes.

Hackability - It should be easy to lash together and script the different portions of this web. MySpace, for example, allows users to embed components from all over the web; Google's AdSense, another example, allows ads to be integrated onto arbitrary web pages. What would you like to hack together, using the web as a base?

Openness - Whether the protocols used are de facto or de-jure, they should either be documented with open specifications or open code. Any entity should be able to implement these standards or use this code to hook into the system, without penalty of patents, copyright of standards, etc.

From Gift Economies to Free Markets - The Open Web should support extreme gift economies, such as open source and Wikis, all the way to traditional free market entities, such as Amazon.com and Google. I call this Freedom of Social Forms; the tent is big enough to support many forms of social and economic organization, including ones we haven't imagined yet.

Third-Party Integration - At all layers of the system third-parties should be able to hook into the system, whether creating web browsers, web servers, web services, etc.

Third-Party Innovation - Parties should be able to innovate and create without asking the powers-that-be for permission.

Civil Society and Discourse - An open web promotes both many-to-many and one-to-many communication, allowing for millions of conversations by millions of people, across a range of conversation modalities.

Two-Way Communication - An Open Web should allow anyone to assume three different roles: Readers, Writers, and Code Hackers. Readers read content, Writers write content, and Code Hackers hack new network services that empower the first two roles.

End-User Usability and Integration - One of the original insights of the web was to bind all of this together with an easy to use web browser that was integrated for ease of use, despite the highly decentralized nature of the web. The Open Web should continue to empower the mainstream rather than the tech elite with easy to use next generation browsers that are highly usable and integrated despite having an open infrastructure. Open should not mean hard to use. Why can't we have the design brilliance of Steve Jobs coupled with the geek openness of Steve Wozniak? Making them an either/or is a false dichotomy.

More info: http://codinginparadise.org/weblog/2008/04/whats-open-web-and-why-is-it-important.html