An init container runs and stops. A sidecar runs and stays, beside the app, for as long as the Pod lives.

Native sidecars

Since Kubernetes 1.29 a sidecar is written as an init container with its own restartPolicy: Always:

spec:
  volumes:
    - name: logs
      emptyDir: {}
  initContainers:
    - name: log-shipper
      image: busybox
      restartPolicy: Always
      command: ["sh", "-c", "while true; do echo shipping logs; sleep 5; done"]
      volumeMounts:
        - name: logs
          mountPath: /var/log/app
  containers:
    - name: web
      image: nginx:1.27
      volumeMounts:
        - name: logs
          mountPath: /var/log/nginx

That single field changes the rules for that container:

Why the order matters

A log shipper should be reading before the app writes. A proxy should be listening before the app takes its first request. Writing the helper as a plain second container gives no such guarantee; writing it as a sidecar does. And when the Pod shuts down, sidecars stop after the app, so the last log line still gets shipped.