Any package available in market are general purpose but if 
any company want to take them there are two ways either 
company should change their system as per the available 
package which is practically not so easy and the second 
option is to modify/customised the software available as 
per the company system which is called customization.
Example : In real time scenario suppose two different  
companies A and B bought Oracle Financial (APPS) GL module.
but company A want only 5 segments while company B has more 
details business process and want 8 segment code 
combination id. this is again part of customization.
one want to work in Multi Org setup while other want it for 
sigle Org , for multi Org also we need to do setups which 
will be specific to that company only.
Balance Sheet is the final output of any organization but 
the presentation of data in balance sheet is diferent for 
different company that is customization only which gives u  
the data in desired format.