Understanding the TeXlive release cycle: What is the meaning of a TeXlive release and is it ever 'finished'?

TeX Live is finalised once per year, as David says in his answer this includes a more-or-less fixed set of binaries. (Binary updates almost never add features, only fix issues, during the year.) At the same time, this release version is made into an ISO for DVD production.

During the year, (macro) package updates happen from this point to the point at which TeX Live is 'frozen'. Thus a TeX Live release does not correspond to any particular macro package version. The 'frozen' version is predictable, and so many people will keep these as a 'reference' and say something like 'tested with TL'XX final'. (For example, I have every TeX Live version from TL'09 onward available on my development system: all are 'final frozen' versions.)

The historic archive contains the final version including all 'in place' updates for the year. If you want the DVD release, grabbing the ISO is the way to go: they are fixed at 'release'.

essentially the binaries only get updated once a year (except for critical updates) but packages and other text files get updated on a rolling basis as they are uploaded to ctan.