What is the meaning of mounting and unmounting?

I have installed Ubuntu 14.04.1 LTS on my system and it is running fine. But whenever I open my 120 GB volume, it says that the disk is mounted and open. What is the meaning of mounting? For instance, on Windows .iso files are mounted as CDDVD. Later on, they can be unmounted using tools like Daemon Tools. What actually means mounting and unmounting? What are the differences between them?
