Shared and Static Libraries

I've been reading An Introduction to GCC: For the GNU Compilers GCC and G++ (also available online for free, from the publisher).  I'm not that experienced with GCC and there's a lot of good information in this book.  It's a little out dated but I believe most still applies to today's versions of GCC.

One part of the book discusses libraries.  I've not really had any experience with libraries and they always seemed to mystify me.  I found this quote to really clear one part of that up:

"External libraries are usually provided in two forms: static libraries and shared libraries. Static libraries are the ‘.a’ files seen earlier. When a program is linked against a static library, the machine code from the object files for any external functions used by the program is copied from the library into the final executable.

Shared libraries are handled with a more advanced form of linking, which makes the executable file smaller. They use the extension ‘.so’, which stands for shared object."

It's simple, straight forward, and now that I understand, I feel silly for previously not understanding libraries.  And besides that, the discussion about LD_LIBRARY_PATH (loader library path) and LIBRARY_PATH (linker library path) is very helpful.  Section 3.1.2 starts a very good explanation.