Difference between Customization and Optimization

Customization is merely a change in the delivered code for attaining the requirement of the client -- it can be adding a new field on the page just to show the payment method/ to show the payment date / changing the label of a field, etc

while Optimizing is in general terms getting the best out of any thing.. it can changes in a process to reduce the time of run, or using the same report for two purposes which reduced the effort in creation of a new report or adding one more search criteria to refine the search is also optimizing the search process..

There is very fine and minute line of difference between Customization and Optimization.
eg: Customer wants to have a report with employee address data with the job data in one report. and he also needs a way where he can run the report for all employees of a department i.e. bulk run option.
Now developer has to give the bulk option, so he is optimizing the codes/ conditions.. and he is doing this in delivered report then it is also a customization.