It really isn't difficult. Just imagine if staff in a bank went round telling everyone how much money their customers had in their accounts.
Customers don't want everyone knowing their business and would be less likely to use your Company if they thought that their details etc were being given out to all and sundry.
Also you should treat information obtained through employment as confidential. Your employer's competitors should not benefit from confidential information.
Also consider the data protection act.
You'd probably get a wider range of answers (as well as appearing more polite) if you gave a bit of background to your question, otherwise it does just look like help with homework.