Banks accepts deposits and loans from customers.
The profits is what it gets from loans less the interest it pays
to the depositors.
Incidentally, banks can generate money based on the Capital
Adequacy Ratio (CAR) set by Monetary Authority of Singapore
(MAS).
If you have a deposit of say $100,000, the bank only need retain
10% of $100,000 in physical cash as required by MAS.
Therefore the amount of money generated out of your deposit of
$100,000 is approximately $900,000 (i.e. 90,000 / 0.10).
If Mr.A deposits in the bank $100,000, the bank loans out
$90,000 to Mr.B (even if Mr.B spends all his money, the person who
made the money will
need to deposit the money into the bank), so the $90,000 ends up
in the bank and the bank will retain $9,000 and loan out
$81,000......................
So ultimately, out of the loans generated of $900,000, the bank
only has $100,000 physical cash to back up.
This principle works on the assumption that not everyone will
withdraw 100% of their money everytime.


Source: http://www.mas.gov.sg/news_room/press_releases/2007/Change_to_Capital_Adequacy_Requirements.html