- "stable" is considered stable enough for production and might receive new features, which have been tested in "testing".
- "long term" will only receive bugfixes in minor version updates. When stable is out for a while it may become the new "long term" and only receive bugfixes afterwards.
I personally only deploy "long term" on my production sites and use "stable" for one power user, who agreed to being the lab rat for new features. "testing" is only used in my lab setup and it can totally happen, that I have to netinstall "testing" on a regular basis, because someone screwed up.
Note: NEVER ever use "testing" in prodution! Never. never. really. Never! No, don't even think about it.