A list is a bag of things. For example, we might look out the window and list the colors of passing cars:
MyList: blue; yellow; black; white; blue
We can examine this and say there were five cars, and that two of them were black. We can also say that the first car was blue.
A set is a collection that has no duplicates. If we watched the passing cars and kept the colors in a set, we might have
MySet: blue; yellow; black; white
Now, we can see that there are four distinct colors. (That might be easy to see from the list if it’s a small list, be it would be tedious to learn in we had 1000 cars.). We don’t know how many cars there were — only how many different colors.