There are no specific ways of defining the difference between standard and customized. Any process defined by SAP when you change it to match your requirement is called customized.
For instance when you introduce a BAdI or User Exit to carry out certain check or changes while any transaction then the process is called customized.
Also, imagine a requirement by your client saying he needs a specific report which is not available and you create a new one. This as well will be classified in that category.
/Manoj