The Deep Web is simply content not searchable via Search Engines and are hard to find unless you know the direct address.
The "Deep Web" you speak of, is a set of such sites only accessible by using TOR to provide anonymity as much as possible. The Deep Web uses TOR to provide a 'not' DNS for IP to Web address translation via something called a .onion which your browser cannot regularly open.
Once on a TOR network, you can access a .onion the same way you would a .com. Once you know of a .onion to visit, though there is a 'deep web' form of google not even a quarter of the sites are indexed on it you can simply visit it and you will be on the "Deep Web".
Complicated topics explained, simply.