Here you should find a pretty comprehensive list of terms, phrases, information and other goodies associated with the Guard. Please let me know if you think something should be added or if you have questions.
First, visit Defending Humanity. His Terms and Definitions page will save your life.
Then, visit Admiral Drax - you won't find a nicer bloke. His conversions are tremendous, his armies are fun, and he loves those commissioned to serve him.