The Deep Web, in simple words, is that part of the world-wide-web that cannot be indexed by search engines. Contrary to what most people think, the Deep Web and the Dark Web are not truly interchangeable terms. The Deep Web generally refers to any content residing behind log-in pages; examples are your Facebook messages page, banking account page or username/passwords authentication web servers. It also refers to content that, while accessible to anyone, is deliberately made hidden from search engines; mainly using the robots.txt file.
On the other hand, the Dark Web refers to pages that are stored in private and encrypted networks/protocols; the most popular of these networks/protocols is the Tor network. These pages, usually having a .onion suffix URL, cannot be indexed by traditional search engines (such as Google and Bing) and require special search engines that operate via the same protocol to be crawled.
One such special search engine and arguably the most known is Torch.
Torch is one of the oldest and most well-known darknet search engines. It currently has over 290,000 indexed onion pages and serves over 80,000 requests per day. It’s very easy to use and only consists of a search box to enter your search terms. Moreover, the search engine offers advertisement banners like the ones you see in the screenshot above with fair prices. Most of the profit goes to support and maintain the servers to keep the service working for everyone.
As stated by the Torch team, Torch is always indexing and crawling Tor onion pages nonstop. It also revisits already indexed pages to eliminate defunct ones from its databases
With such a huge number of indexed pages, Torch is easily considered by many as one of the most efficient and useful Dark Web search engines.