Firebase Cloud Messaging is a cross-platform messaging solution that allows sending messages at no cost. It supports notifications or data messages up to 4KB in size and versatile targeting. Notifications are displayed to the user, while data messages are received in the background. FCM supports notification payloads for display and click actions, as well as custom data payloads. Proper handling of notifications in the foreground and background is required. Token management and missing notifications can be issues to address. Capabilities vary across platforms for notifications, permissions, and upstream messaging.