libz.dylib versus libz.1.2.3.dylib versus libz.1.2.5.dylib
The OS often includes many versions of dynamic libraries. These are used by different programs depending on which library they were compiled against at their compile time, but when you compile you want to link against the version that correspond to the installed headers which you are including/importing into your source code.
The libz.dylib will be a link to the same version that your installed headers use.
Say you have 2 versions libXYZ.1.dylib and libXYZ.2.dylib, libXYZ.dylib is a link to libXYZ.2.dylib and libXYZ.1.dylib is a legacy lib that is also available in the OS for apps compiled and distributed before libXYZ.2.dylib was released. The libXYZ.1.dylib has been included in the SDK because there can be old frameworks that still want to be linked against the old version.
The two versions might have very similar interfaces in the header so you won't see any real differences when you compile and run, but in future versions the older versions might get removed and new ones added which will make your project break when linking.
If I understand it right, the linker will dereference file links so it will find the right version and keep that dylib name and dynamically link against that when the app starts. So the libz.dylib won't be the path used (more than at compile time).
I see this in my Xcode installation in the 4.3 SDK
/Developer/.../SDKs/iPhoneOS4.3.sdk/usr/include/zlib.h
/* zlib.h -- interface of the 'zlib' general purpose compression library version 1.2.3, July 18th, 2005 Copyright (C) 1995-2005 Jean-loup Gailly and Mark Adlerlibz.dylib
/Developer/.../SDKs/iPhoneOS4.3.sdk/usr/lib/libz.dylib -> libz.1.2.3.dylib
You can easily see in the finder how they work. In XCode, "Show in Finder" one of the libraries. Now click once on libz.dylib and "Get Info". You'll see that "Original" is:
/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.0.sdk/usr/lib/libz.1.2.5.dylib (as of XCode4.2 with iOS 5 SDK)
So it's a symbolic link to version 1.2.5 for now. In the future it will update to the latest 1.x.x. You can examine all of the various versions this way.