Depending on how the information is accessed in the web, it is possible to divide this info in 3 categories:
The Surface Web
The Deep Web
The Surface Web
is composed of that information (web pages, documents, ...) that can be indexed by search engines like Google, Bing , Yahoo and Yandex.
To understand the concept of deep web you should know how search engines work. The main content indexed by search engines depends on links they find on web pages. They work like any internet user when is browsing a open traditional news, blog or any other site that contains links. For instance, Wikipedia, BBC, NY Times, etc. The user clicks on links and is directed to another web page. He / she can click again and reach another web page in the same site (an internal link) or in other site (an external link). Doing this indefinitely, search engines can index (record and classify the information) and show to anyone who uses its search page. The content that is accessible by search engines is the so called 'Surface Web'.
We can define Deep Web
as anything that a search engine can’t find. Most of the web's information is hidden on sites. Traditional search engines cannot see or retrieve content in the deep web. There are many examples which can be used to explain what is deep web. Let's take the example of an bank web site. You or a search engine can visit its home page, navigate through its menu, visit some open pages, etc. But there are informations you can't access because it is in a restricted area. This data is protected by passwords or other method that makes that inaccessible by search engines. As a second example your Gmail. The content of this website is not crawlable by external bots once to access yor account someone must type your login and password. We can also cite other resources, online banking, video on demand, you data on a online store and many more.