Version and Release

["c.holper@ades.at" <c.holper@ades.at>]

Hi!

I am not completely new to Debian but I never really understood the 
version-system.

I understand the versions have nicknames (8 = jessie, 7 = wheezy, 6 = 
squeeze, ...).
I also understand there are 3 releases of every version (stable, 
testing, unstable) and
it is clear that stable has the oldest but most bugfree version of 
software and unstable the opposit.

But it seems there is some living connection between the current testing 
and the future version...
And what is "sid"?

I downloades 8 testing (some months ago) and in my apt-sources is 
jessie. Would it be something else if I chose stable??
How can I see it in apt?
Is there a better way to determine the release beside lsb_relsease?
/etc/debian_version is pretty clear but only the verion is inside.

Please clearifiy for me the impact of the the once chosen release in 
connection with the current and future version.
Big thankyou!

Cheers,
Chris