Stateless failover means the backup starts fresh. Existing connections are dropped. Clients must reconnect. This is simpler but disruptive.
Stateful failover synchronizes session data between primary and backup. When failover happens, connections continue without interruption.
Firewalls and load balancers often need stateful failover. They track TCP sessions, NAT translations, and connection states. Without synchronization, clients experience broken connections.
Stateful failover requires bandwidth for synchronization. It adds complexity but provides smooth experience for users.