Docker is an open source project designed to help with application deployment using software containers. This quote is from the official Docker page:
"Docker containers wrap a piece of software in a complete filesystem that contains everything needed to run: code, runtime, system tools, system libraries - anything that can be installed on a server. This guarantees that the software will always run the same, regardless of its environment."
Docker, therefore, in a similar way as virtualization, allows packaging an application into an image that can be run everywhere.
