Container
A practical DevOps definition, with an example you can use.
An isolated process with the application files and dependencies it needs to run.
An example you can picture
You start two containers from the same web-server image. Each has its own process lifecycle and writable layer, while the image supplies the starting filesystem.
The distinction that matters
A container is not a complete virtual machine. Containers share a host kernel; isolation and portability still depend on runtime configuration and compatibility.
Put it into practice
Run a small container in your lab, inspect its process and port mapping, then stop it. Explain which data lives in the writable layer and which data would need persistent storage.
Connect the ideas: docker image and virtual machine. For the bigger picture, read the related learning guide.
Reference
Read the primary documentation for the underlying concept and further detail.