A Virtual Machine is an image that runs on top of a physical server. We can create multiple Virtual Machines on top of a physical server. Each Virtual Machine needs a Guest operating system to function, this makes is heavier since each Guest OS needs Memory, Disk, and CPU to operate. 

A Docker Container can be seen as the next version/improvement to a Virtual Machine. Each Virtual Machine in a server needs a separate Guest operating system to function, but all the Docker Containers in a server use the same underlying Operating System of the server, hence Dockers are lightweight when compared to Virtual Machines. Also, Dockers are faster to spin up and execute when compared to Virtual Machines.