Vivaldi is a browser suite, offering an email client, a feed reader, a calendar, and a notes organiser in addition to its core browser functionality. It’s known as a browser aimed at “power users”, offering a high degree of customisability in its UI, and some innovative tab management features. Some of its other perks include an in-built ad blocker, webpage translation, and the ability to annotate webpages.
The company Vivaldi was founded by ex-employees of Opera Software, who were annoyed with the direction their old employer had decided to take. They started by offering a community site, with free webmail and blogs for members, to replace My Opera after it was shut down in 2014. The Vivaldi Community is still around and apparently thriving today. The browser itself was officially released in 2016, built upon Chromium with a custom (closed-source) UI. The intended audience was the same kind of person to whom Opera itself once appealed: power users.
Vivaldi have published a blog post explaining why they have chosen not to open-source their UI (which is the one component that is closed-source), which is basically that they don’t want someone to be able to fork their browser, invest minimal technical efforts of their own, then overshadow them with well-placed marketing at a time when Vivaldi is still a pretty small and vulnerable company. (Or else, devalue the brand with a poorly-maintained fork.)
The browser doesn’t track its users or include any kind of telemetry. Their business model is to do partnership deals with search engines (every search engine pre-installed in their browser, except for Google, is one they have done a deal with) and also with some other companies in exchange for browsers being pre-installed with bookmarks to those companies’ sites. They have published on their blog that they will never get into the cryptocurrency space, unlike *cough* certain other browsers.