You mean the tabs in the staff control panel?
The main place you will find them is in include/class.nav.php
There is, however, also code for tabs spread out across other files, so that the active tab is set correctly, and possibly for sub-tabs. It is easy enough to find this code if you use a text editor (such as crimson editor) where you can search a directory for specific code. If you search for something along the lines of $tabs you should be able to find what you are looking for!
