Balancing Engine Parts

[<slowboy@cox.net>]

Hi gang!

Balancing engine parts is the topic for me today. Is there a website that may 
have a detailed explanation of what is involed with balancing parts?

I know what balancing does for items that spin but I am curious why we would 
balance connecting rods and then balance them to the crank.

I was also told to balance my connecting rods as I was only able to obtain them 
new from one manufacturer but four different vendors around the country.

Thanks!