Dooble eschews the Chromium base some of the other browsers use and is in fact an original browser, built using the Qt interface and WebKit HTML rendering framework. For those unfamiliar, Qt is a cross-platform toolkit used heavily in the KDE Linux desktop, and WebKit itself originated with KDE as KHTML (which you can still see in user-agent strings, and was later appropriated by Apple for Safari, and Google for Chrome before forking it into its own version called Blink -- but enough history for now!).
This means a couple of things for Dooble: firstly, it has a range of interesting features unique to the browser, and that while fully-featured it isn't quite as streamlined as browsers like Chrome and Firefox. There are a lot of menu options and the style is looking dated now compared to the latest iterations of its popular counterparts, but if you look past this it's a highly-capable browser.
It also requires you to set a passphrase if you wish to save and restore a session, which includes booksmarks, cache, cookies, history and other related data. All of this becomes encrypted with your choice of cipher and hash type to prevent anyone else but the user accessing their respective session data.
Dooble uses the Metager search engine by default (see below) and comes in flavours for Linux, Windows and MacOS X. If there's one fault for Dooble it's the distinct lack of documentation, making it hard for less computer literate users to understand some of the more advanced features that help to set it apart.