Deep web (opposite of surface web) is that portion of WWW which is not indexed by search engine. It is implemented by a special form of http and can be accessed by normal IP address and URL but the content is access controlled. That means the contents are intentionally controlled. Examples are video on demand and online newspapers etc..,
Dark web is synonymously called dark net, free net where illegal activities take place. This is a portion of the deep web.
Several tools like DeepPeep etc have accessed the deep web . Tor (The Onion Router) is one interesting software which can help in accessing some links while preserving personal privacy.
{Source: Wikipedia]