What is Cilium?
Cilium is an open-source CNI (Container Network Interface) for Kubernetes. It uses Linux eBPF technology to provide high-performance networking, security, load balancing, and observability.

Where is it used?
Cilium is commonly used in Kubernetes environments for Pod-to-Pod networking, NetworkPolicy enforcement, Service load balancing, multi-cluster connectivity, and network monitoring. It is especially useful in large-scale cloud-native environments where performance, security, and visibility are important.

Key features: eBPF-based networking, L3/L4/L7 security policies, kube-proxy replacement, encryption with WireGuard/IPsec, and network observability through Hubble.

Alternatives: Calico is a popular feature-rich CNI with strong network-policy capabilities; Flannel is a simpler option mainly focused on Pod networking; Antrea is a Kubernetes-native networking and security solution based on Open vSwitch; and Kube-router provides networking and policy with a relatively lightweight architecture.

In short:
Cilium = Kubernetes networking + security + observability, powered by eBPF.