Difference between user exit, field exit & screen exit
User exits are empty routines that are called within a SAP standard program, where a customer can enhance the code with their own logic.
A field exit allows you to assign a special function to a data element that is triggered every time the value of a field on the screen changes. Screen exits are widely the same as field exits but the do not attach to the data element but to a field within a special screen.
Search in Google for how to:
site:help.sap.com field exitsite:help.sap.com user exit
site:help.sap.com screen exit