Obsidian is a free* notetaking tool similar to one note and such. It uses markdown for formatting and offers good extension support.

I've used it for free for about a year and I'm pretty happy with it.

*Some services are offered only for paid users, I think it's only online storage /sync but I'm not sure.

Like the others were saying it's a note taking tool, it stores notes in a markdown format so you can better own your notes because it's not stored in some weird format and is just plain text.

It has a few extra features primarily from its community plugins but there's also the graph view and more recently added, the canvas/whiteboard view. Where the former provides a graph of how your notes connect when you inter-link them like your own personal wiki; and the latter aims to provide a 2D visual board to put notes, links, and other content and connect them.

Also like the others said backup/sync is a paid feature, but there are a few quite simple ways to sync with free services such as Google Drive, iCloud, Dropbox, or Syncthing (device to device sync)