For a very straightforward description of heraldry rules, here's the SCA's standardized way of doing things:
http://heraldry.sca.org/primer/Historically, the rules changed a lot over the years, and varied between countries. But many groups (such as the SCA, and The Order of the Marshal, etc) will use this sort of standardized set of rules that are derived from historical English rules. I really like that "primer" from the SCA since it's a nice concise place to get an overview of the rules.
There are many websites that show what the individual charges, colors, divisions, and so forth can mean. Here's an example from some quick googling:
http://www.dksdesigns.net/symbolism.htmlHeraldry on shields started out as a simple way to paint up the shield to make a knight recognizable on the field. Up until the 13th century, it tended to be simple geometric patterns. During the 13th they started to get more complex, and particularly in the 14th century you tend to see a lot more animals and objects used as charges. By the 15th century, heraldic designs could by much more complex, with "marshaling" to combine the arms of multiple families, as the system had become more codified and more meaningful beyond just battlefield use.
