Sets are conventionally denoted with capital letters. Sets A and B are equal if and only if they have precisely the same elements.[2]
There is the image popular, that sets are like boxes containing their elements. But there is a huge difference between boxes and sets. While boxes don't change their identity when objects are removed from or added to them, sets change their identity when their elements change. So its better to have the image of a set as the content of an imaginary box: