Dark Web is a term for all of the content on the Internet that is not indexed by search engines like google and bing. These include information that are behind strong encryption mechanisms and hence .onion websites are not directly accessible on public internet.
Orfox is built from the same source code as Tor Browser (which is built upon Firefox), but with a few minor modifications to the privacy enhancing features to make them compatible with Firefox for Android and the Android operating system.
Orbot is a free proxy app that empowers other apps to use the internet more securely. Orbot uses Tor to encrypt your Internet traffic and then hides it by bouncing through a series of computers around the world.
Fire.onion lets you browse the web anonymously by routing all traffic over a series of randomly-selected proxy servers using a technique called onion routing.