1. Shared Objects

Shared objects (a.k.a. dynamically loadable libraries) are the most
common way to ship code; as you may recall, OpenSolaris deliberately 
removed static compilation from options available to developers
(https://blogs.oracle.com/rie/entry/static_linking_where_did_it)

On Linux and OpenSolaris/Illumos you will find shared objects
needed by a dynamically linked executable with ldd:

$ which ls
      /usr/bin/ls

$ ldd /usr/bin/ls
	      linux-vdso.so.1 =>  (0x00007fffc8dd5000)
	      libselinux.so.1 => /lib64/libselinux.so.1 (0x00000032d7800000)
	      librt.so.1 => /lib64/librt.so.1 (0x00000032d6800000)
	      libcap.so.2 => /lib64/libcap.so.2 (0x00000032d9c00000)
	      libacl.so.1 => /lib64/libacl.so.1 (0x00000032f0800000)
	      libc.so.6 => /lib64/libc.so.6 (0x00000032d5800000)
	      libpcre.so.1 => /lib64/libpcre.so.1 (0x00000032d7400000)
	      libdl.so.2 => /lib64/libdl.so.2 (0x00000032d6400000)
	      /lib64/ld-linux-x86-64.so.2 (0x00000032d5400000)
	      libpthread.so.0 => /lib64/libpthread.so.0 (0x00000032d6000000)
	      libattr.so.1 => /lib64/libattr.so.1 (0x00000032e9000000)

(taken on tahoe.cs.dartmouth.edu, 64bit Linux)

Skim "man ld.so" to know what options and configurations exist on
a Unix system.

On Windows, a similar role is played by .DLL files (Windows Portable Executable
format, PE, is a cousin of ELF, but includes many more types of sections, such
as the icon for the executable in the form of a simple bitmap). 

MacOS X uses the Mach-O format for executables; shared libraries are in .dylib
files. Apple's "otool" has options to parse these files and find dependencies 
(but a verison of objdump, called gobjdump, also exists in /opt/local/bin/gobjdump
and can do disassembly and section dumps to hex, see "man gobjdump").

$ file /bin/ls
/bin/ls: Mach-O fat file with 2 architectures

$ otool -L /bin/ls
/bin/ls:
	/usr/lib/libncurses.5.4.dylib (compatibility version 5.4.0, current version 5.4.0)
	/usr/lib/libutil.dylib (compatibility version 1.0.0, current version 1.0.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 159.1.0)
  
(works kind of like "ldd")

Examine synamic symbols provided by a shared object with "nm -D" on Linux & Illumos,
"nm" on MacOS X. 

2. Building shared objects

Read "Shared objects for the object disoriented! How to write
dynamically loadable libraries" by Ashish Bansal:
http://www.ibm.com/developerworks/library/l-shobj/ 

Have a look at "Advanced Linux Programming" Chapter 1, Sec. 1.2 and
Chapter 2, Sec. 2.3. "Writing and Using Libraries". The full book is
freely available:
http://www.advancedlinuxprogramming.com/alp-folder/

GNU toolchain contains Libtool, which collects the many necessary options
for building shared objects across different systems, making it easier
to write build scripts that work on many systems:
http://www.gnu.org/software/libtool/manual/libtool.html