Well, the notion stable/unstable is not very good - even the stable series are not bug free (and nothing is :). But the stable series are in feature freeze and there is no active development made on them - just bug fixes and small features being added.
It's not really a "feature freeze" if small features are added :)
The stable series is intended to see only changes that are already stable. New drivers may be backported from 2.6, or merged from vendors. In some extreme cases, even major changes get made to the "stable" series if Linux thinks it'll improve things. The sheduler change in 2.4.10 was an example of that.
