What other containerization systems exist besides Docker?

Besides Docker, there are other containerization systems:

• Podman — an alternative to Docker, does not require a daemon, runs without root, well integrated with systemd.
• containerd — a low-level container runtime used inside Docker but can also operate independently.
• CRI-O — a container runtime focused on Kubernetes, implements the CRI interface.
• LXC (Linux Containers) — a more "low-level" containerization system that uses cgroups and namespaces directly, often used for creating lightweight virtual environments.

All these systems provide application isolation but differ in architecture, level of abstraction, and use cases.