You can think of it like an encrypted messenger that looks and feels like a social network. Or you can think of Circles as a social network app where every post is encrypted from end to end.
Every post in Circles is encrypted from your device all the way to your friends' devices. The only people who can see the messages and photos that you share are the people who you have specifically invited to join your social circles and groups.
Circles is built on Matrix. It uses the same E2E security protocols, Olm and Megolm, that Matrix uses for its encrypted group chat. Circles does not attempt to "roll its own" encryption code. Instead, it uses the Matrix iOS SDK for all security and encryption related functions.
Circles for iOS should run on any device with iOS 14+. Building a version of Circles for Android devices is one of our top priorities for the future.
A group in the Circles app works pretty much like a group anywhere else. It has a well-defined set of members, and everyone who's in the group is in the same group with everyone else. Everyone in the group can see everything posted in the group.