Most of us use the surface level of the web, the so-called Surface Web (aka Visible/Public/Open Web) on a daily basis. The Surface Web is basically anything that can be found using a search engine like Yahoo, Bing, or Google.
To find out, watch this 5 min video.
Under the easily accessible Surface Web, there are millions and millions of files full of financial data, photos and other material that's not freely accessible. This layer comprises the vast majority of the total web and is called 'Deep Web'.
The Deep Web is comprised of content that can't be indexed by search engines - you can’t google it or find it with any other search engine.
Basically, Deep Web content is found anytime when you do a search directly in a website or when you log into your account on a website. Google's web crawlers can’t trace the pages behind these website search boxes.
This means that nearly all of the information that university libraries offer belongs to the Deep Web. Only those who study or work at a well-funded university can access this information. As a result, a large portion of the research literature isn't available to potential users (e.g. students and researchers from universities located in lower income countries).
And then there's the Dark Web. This is the most hidden part of the web that is inaccessible through regular web browsers. It is most known for criminal activity (e.g., identity theft, credit card scams, and worse).
Open access is a broad, international academic movement aiming at free online access to scholarly information. The open access publishing model differs from the traditional subscription model in which readers have access to scholarly information by paying a subscription (usually via libraries). When content is available open access, the icon is often displayed - but not always!
Academics can make their articles open access by:
depositing their article (as accepted for publication by a journal) into an institutional repository, making it freely accessible for everyone. An institutional repository is a digital archive for publications written by academics employed by a university. This type of OA is often referred to as 'self-archiving'.