[O] conceptual TODO help?

[scrawler]

Hi guys, I have a few questions about todo states. I have these
keywords defined:

#+TODO: Do Doing DoBy DoLater | Did DidNot DidNever 
#+TAGS: @Beverly(y) @Bionca(a)

I'd like to have a better understanding of the TODO sequence for any
given task, assignment, or project. The states Do and Did are pretty
obviously just TODO and DONE renamed. I can maybe add a
DEADLINE:<2012-07-28 Sat> for the DoBy heading, and a
SCHEDULED:<2012-07-28 Sat> for DoLater, but what can or should I do
with Doing? Should I just leave it alone and look at it?

I think I've seen TODO keywords defined (in the manual? list emails?)
like "feedback", etc, but it seems like if I have a basic but
comprehensive sequence I can put that kind of stuff in tags
(Incidentally, what are the @ signs in front of tags for?).

Are there any keywords I should add or remove? Should I get rid of
DidNot and DidNever (implying not yet and never will)? My keywords are
arbitrary and silly, but I'd like to better understand the states they
label.

Thanks, I hope I was clear, and sorry for the "basic question" noise.