Two box plots have the same median and equally long whiskers. if one box plot has a longer box than the other box plot, what does this tell you about the difference between the data sets?

The first line as you know is the lowest value
The second line (start of box) is the lower quartile
The Middle line is the median
The fourth line (End of box) is the upper quartile 
The last line is the highest value
A box plot with a longer box shows that interquartile range is likely to be larger suggesting more of a spread in data.