The most commonly used web browsers on a Mac running OS X are: Safari and Firefox.   If you wish to delete the cookies stored on your mac follow these directions:

A.  Safari version 6.0.3:

1.  Open ‘Safari’ from the main menu;
2.  Click on ‘Preferences’ or press the  key;
3.  Click the ‘Privacy’ tab with the silhoutte icon;

Safari Privacy Tab image
4.  Click on ‘Remove All Website Data' or select `details' to remove individual cookies

Remember that removing cookies in Safari may change or remove them in other applications such as Dashboard.To

B.  To Delete Cookies in Firefox version 19.0.2 on a Mac:

To Delete Cookies in Firefox 19.0.2 on Mac

1.  Start ‘Firefox’ from the main menu
2.  Select ‘Preferences’ and click or press the  key;
3.  Select ‘Privacy’ by clicking on the mask image;


4.  Choose ‘Show Cookies’ and choose ‘Remove Cookies’ or 'Remove All Cookies';

Some people view 'cookies' as an evil corporate snooping invention but used correctly they can be a great automation tool.  Firefox, for example, can remember the 'userid' and 'login password' used on various websites.  These days it is not uncommon to use dozens of different userid+password combinations.  This can be a real benefit for those who use a large number of accounts.