Nextcloud is a cloud-based "file sync and share software". It offers

syncing of OS-based directories (Linux, Windows, macOS) with the Nextcloud server ("Desktop Sync client") [1]
a browser-based desktop (the "Web interface") with various "apps" (including "groupware").
additional functionality via an app store [2]

test instances

apps (basic funtionality

Files: the (synchronized) files on the server
- files can have tags and can be shared
- viewers for videos and images
Photos: organization of images
Activities: a log of changes that happened in the apps -- sometimes links are missing to changes, it is not possible to directly jump to them
Mail
Contacts
Calendars
Deck: Allows note-taking, hierarchical organization into deck -> board -> stack -> card
- Card: has users, description, attachments, timeline, tags
project: collection of items (files, chats, etc.)

Talk: Screensharing, online meetings & web conferencing [3]
Polls: kinda like Doodle [4]
OnlyOffice: browser-based text processing
- " types of supported formats (ODT, DOC, RTF, EPUB, MHT, HTML, HTM, ODS, XLS, CSV, ODP, PPT, DOTX, XLTX, POTX, OTT, OTS, OTP, and PDF-A) are processed with inner conversion to DOCX, XLSX or PPTX" [5]
...