What is difference between 'Merge Data table' and 'Join data table' activity?

What is difference between ‘Merge Data table’ and ‘Join data table’ activity?

merge datatable is used e.g. to copy the datatable from a second datatable to distination datatable

@ppr Thanks for your input. I am just trying to understand what could be possible difference between these 2 activities ? when to which specific activity?
Actually this question is being asked in interview repeatedly so seeking for an answer?

in that case the best would you implement two cases so you will understand and will get practical experience.

Thanks @ppr.
Actually I implemented both the scenarios and I am able to conclude above is the difference as per explanation.

I am still not clear about exact purpose of these activities when to use which one?
In real time scenario, based on what parameters we can decide which activity to use ?
Please help.