A Standard channel is the default: anyone already in the Team can see and post in it, and its files live in the Team's shared SharePoint site. A Private channel carves out a smaller, restricted conversation, like a leadership discussion or a sensitive client matter, with its own separate, hidden SharePoint site that only its members can reach, even though it technically lives inside the same larger Team.
Shared channels solve a different problem: working directly with an external partner, vendor, or another internal Team without giving them a guest account in your whole organization. Built on Microsoft's cross-tenant connected-experience model, a Shared channel lets both sides collaborate in one place while keeping their own identities and permissions intact.