difference between User Exits and Customer-Exits

Hi

What is the difference between User Exit and Customer

exit?

How to find User Exits?

How to find Customer Exits?

How to Use User Exits?

How to Use Customer Exits?

If i want to disable some fields in standard trans.

based on data, which one is used and how?

I am confused between these two can any one answer

the above questions?

Thanks & Regards,

Abhimanyu.L