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+# Foundations
+* Container: "lightweight" VM.
+* Docker: enables containerization.
+* Hypervisor: layer between os and application to provide virtualization layer.
+  VMs run on top of hypervisor.
+  - Type-1: runs on bare metal, mediates between hardware and OS. E.g.
+    Citrix/Xen server, Microsoft Hyper-V.
+  - Type-2: runs on top of OS, mediates between OS below it and OS running on
+    top of it. E.g. Oracle Virtual Box.
+  Docker is thus *similar* to a type-2 hypervisor.
+* A container does not run a full-blown OS, tries to reuse OS running under
+  the docker engine.
+* Container gets hardware resources on-demand, only when it needs to use them.
+  Whenever it does not need resources they are freed back into the OS under the
+  docker engine.
+* Docker Hub: world's largest library and community for container images.
+* All dependencies of a container get installed from docker hub when docker
+  engine runs the container.
+* Docker image: a static file that contains everything needed to run an
+  application, including the application code, libraries, dependencies, and the
+  runtime environment. It's like a snapshot of a container that, when executed,
+  creates a Docker container.
+* Docker platform was designed natively for Linux.
+* The container shares the Linux kernel with the underlying OS.
+* When Docker platform is running on Windows and has to run a Linux container,
+  it runs it using a LinuxKit based virtual machine running on Hyper-V.
+* Advantages of Docker:
+  - No pre-allocation of RAM.
+  - Decreases costs by allowing sharing of resources between containers.
+  - Re-usability by sharing of images. Note: images are immutable. An image is
+    a "snapshot" of a container.
+* Disadvantages of Docker:
+  - Lack of cross-platform support.
+  - Docker is limited to applications that do not have GUI.
+  - Difficult to manage large amount of containers.
+  - It is recommended to run images on the OS they were created in.
+* Images are organized in layers: every change introduced is part of a layer.
+  Each layer represents a specific modification to the file system (inside the
+  container), such as adding a new file or modifying an existing one. Once a
+  layer is created, it becomes immutable. An image can be gotten from Docker
+  Hub, generated from a Docker file, and generated from a running container.
+* Ecosystem:
+  - Docker daemon/server/engine: A persistent background process that manages
+    Docker images, containers, networks, and storage volumes. The Docker daemon
+    constantly listens for Docker API requests and processes them.
+  - Docker client: provides a command line interface (CLI) that allows you to
+    issue build, run, and stop application commands to a Docker daemon.
+  - Docker compose: a tool that helps you define and share multi-container
+    applications.
+  - Docker host: the physical hardware on which docker is being run.
+
+# Practicals
+* `docker images`: list images present in local system
+* `docker search`: search images on Docker Hub
+* `docker pull`: download images from Docker Hub
+* `docker ps`: process status, lists info about running containers
+* `docker run`: runs a container from an image, login's into it's shell
+* `docker start`: starts a container from an image, doesn't login into it's shell
+* `docker attach`: login into a running container's shell
+* `docker stop`: stop a started container
+* `docker rm`: delete a stopped container