The Deep Web

The Deep Web is a) the entire World Wide Web or b) the parts of the web who are there, but not visible when using search-engines.

The Web

Most people experience the World Wide Web using search engines.

• The indexable web are the parts of the web the crawlers these search engines are "allowed" (or not restricted from using standards such as robots.txt, password protected areas, etc) to index.
• The visible web is what's left of the The indexable web after the search engines have censored away those parts of The indexable web you're not allowed to see.
• The Deep Web is the parts of the web who are are not visible to "most people" because the search engine corporations can't OR won't make them a part of their search index. Most people have no idea it's there.

There's also The Hidden Web. This includes Tor's Location hidden services (You can't access them without Tor) and sites hosted on Freenet's distributed file storage system.