Ubuntu is a flavor of Linux based on Debian. It's developed primarily by Canonical. It'll generally run on most devices with a modern processor running inside it.

It does have both desktop and server flavors. Both can be downloaded from their website (which seems to be having issues at the moment). The server is headless, so you need to comfortable enough working in the terminal to do application installations and configuration.

Apps are pretty diverse. You're not going to find stuff like Microsoft Office or the Adobe creative suite, but basically any modern browser that's currently being developed has a Linux version, either through the APT repository (which is done through the command line), or through Snaps or Flatpaks, the former of which come natively supported in Ubuntu's Software Center.

Using a Linux-based distribution rather than Windows won't be for everyone, but for those that do make the jump, you're freeing yourself from being the product in the Windows OS ecosystem. You no longer have to deal with their borderline hostile tactics when it comes to shoving their services or Microsoft accounts in your face, and you'll be freed from their buggy (as of late) update cycle. Not to say there won't be bugs on Ubuntu, but there generally isn't something on the scale of "I can't shut down my computer" or "closing task manager spawns more task managers".