These are generally collected in Includes and attached to the standard program as PERFORM's.
You can fill them with your own source code between the FORM......ENDFORM part which is present in the includes. All user exits start with the word USEREXIT_*
Example :
Finding and implementing userexits :
Goto SE80 --> Select Package VMOD --> Press enter --> You will find all the user exits in SD
For implementation just proceed by writing your custom code in the FORM.....ENDFORM part .
It requires sap access key to implement the user exits.