What kinds of iphone functions can be over rided and how can i find and look at these functions? Or where can i look at them?
You can use class-dump-z to dump the class headers from a binary. For example, you could class dump the headers from SpringBoard to view what functions you can hook.
I recommend using Theos to write your tweaks. It's a really nice platform that is easy to use, and is also well documented.
iphonedevwiki.net is a good wiki for all the unofficial development information.
Hope this was of some use!
Josh
Theos: http://iphonedevwiki.net/index.php/Theos/Getting_Started
class-dump-z: http://code.google.com/p/networkpx/wiki/class_dump_z