Most of us are experienced with virtual machines under Virtualbox, VMware, xen and other technologies allowing us to run an OS (Operating System) within our real OS. The theory behind Docker is similar to the one behind virtual machines. A virtual machine allowed us to run different virtual servers within a single physical device allowing us to isolate services or applications (if a server fails the rest keeps working) and save resources (one hardware instead of many). Virtual machines were revolutionary for sysadmins as it is Docker now.