In break common fields are deleted  whereas in master-
detail , we declare certain entity as a master to get the 
detailed information or report in this case the table 
format is changed. 

In Section(Master Detail) you can have two or more 
different crosstabs or tables(hori/fin) eg. orders, 
customer, products etc
Where as In break one block is break into groups and 
subtotal is shown eg customer can be broken into 
country/state/city

In Section(Master Detail) you can have charts for each 
section or detail. can have chart and table combination as 
well.
Whereas in Breaks u cannot inseart a chart for each of the 
groups formed out of break

Section:the grouped value appears as a header outside the 
block,and groups the entire report.

Break:the grouped value appears as a header within the 
block.