break removes duplictes but the samething cannot be done by 
section.
break displays data within the same cell content and 
sectioing appears outside the grid.

When you do any arithmetic operation on break say sum or 
count, you can see the sum for individual block and for all 
the blocks in bottom.

In Section it performs operation only on individual block.

This is the main difference I have seen except look and 
feel.

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