> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # What are Containers > Explains Linux containers, kernel isolation using namespaces and cgroups, benefits like portability and efficiency, and the container ecosystem including Docker, containerd, runc and OCI standards. Containers are lightweight, isolated user-space environments provided by the host operating system. Each container can run its own processes, expose its own network interfaces, and mount filesystems independently — much like a virtual machine — but all containers on a host share the same OS kernel. This isolation is implemented by Linux kernel primitives such as namespaces (process, network, mount, user) and cgroups (resource control for CPU, memory, I/O). Containers provide process-level isolation on top of a shared kernel — they are not full virtual machines with separate kernels. A presentation slide titled "What Is A Container?" shows four illustrated containers labeled with "Processes," "Network," and "Mounts" above a purple "OS Kernel" bar. A presenter wearing a KodeKloud shirt gestures in the foreground. Why this matters * Lower overhead: Containers share the kernel, so they require less CPU, memory, and disk than full VMs. * Faster startup: Containers can start in seconds because they don't boot a separate kernel. * Portability: Container images bundle application code and runtime dependencies, making them portable across environments that run the same container runtime. Linux kernel features that enable containers | Kernel feature | Purpose | Example | | - | -: | - | | Namespaces | Isolate kernel resources — processes, network, mounts, and users | Process PID isolation with `pid` namespace | | cgroups (control groups) | Limit and account for resource usage — CPU, memory, block I/O | Restrict memory with `memory.limit_in_bytes` | | chroot / mount namespaces | Provide per-container filesystem views and mounts | Mount container root filesystem without affecting host | Container runtime and image ecosystem Linux container technology has evolved through multiple implementations and standards. Below is a compact overview of notable projects and how they relate: | Project | Role | Notes | | - | -: | - | | [LXC](https://linuxcontainers.org/lxc/) | Early Linux containers implementation | Lightweight OS-level virtualization predating modern container tooling | | [LXD](https://linuxcontainers.org/lxd/) | System container manager | Higher-level manager built on LXC for system containers | | [LXCFS](https://linuxcontainers.org/lxcfs/) | Filesystem helpers for container visibility | Improves /proc and other pseudo-filesystem behavior inside containers | | [Docker](https://www.docker.com/) | Developer-friendly container tooling | Initially used LXC, later introduced its own runtime stack | | `libcontainer` → `runc` | Low-level OCI runtime | `runc` is the reference implementation of the Open Container Initiative (OCI) runtime spec | | [containerd](https://containerd.io/) | Container runtime daemon | Manages container lifecycle; used by modern Docker with `runc` for execution | | OCI (Open Container Initiative) | Standardization body | Defines runtime and image format specifications for interoperability | A short history and modern stack * Early: LXC provided the initial Linux container primitives. * Mid: Docker emerged as an easy-to-use tool for building and distributing container images; it originally used LXC. * Later: Docker moved to `libcontainer`, which evolved into `runc`. The Open Container Initiative (OCI) standardized runtime and image formats. * Today: Docker typically uses `containerd` (daemon) + `runc` (OCI runtime) under the hood to create and manage containers. If you want to work directly with low-level primitives, you can explore LXC/LXD. For application development and distribution, Docker and OCI-compliant runtimes (containerd + runc) are the common choice. Why use Docker (or similar tooling)? * Simplifies image creation and distribution with registries. * Provides a high-level CLI and API for building, running, and managing containers. * Abstracts complex setup of namespaces, cgroups, and filesystem mounts so developers can focus on applications. Further reading and references * [Open Container Initiative (OCI)](https://opencontainers.org) — runtime and image format specifications * [Docker](https://www.docker.com/) — container platform for building and running containerized apps * [containerd](https://containerd.io/) — runtime daemon used by Docker and other platforms * [LXC / LXD](https://linuxcontainers.org/) — alternative system container technologies This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.