Mach-O

from Wikipedia:

Mach-O, short for Mach object file format, is a file format for executables, object code, shared libraries, dynamically-loaded code, and core dumps. A derivation of the a.out format, Mach-O offered more extensibility and faster access to information in the symbol table.

File Format Reference can be found here

Following tools are used in Mac OS X to view Mach-O files:

otool - object file displaying tool

nm - display name list (symbol table)