The [About](http://tex.stackexchange.com/about) should cover this, but for the avoidance of doubt:

 1. TeX StackExchange (TeX-sx) is for questions about TeX, LaTeX, ConTeXt and related topics
 2. [Meta](http://tex.stackexchange.com/help/whats-meta) TeX-sx is for questions about TeX-sx itself
 3.  Meta StackOverflow is for questions about the StackExchange network, as well as questions specific to StackOverflow
 4. [StackOverflow](http://stackexchange.com/about) is a general site focussed on programming, from which many spin-out sites have grown, including TeX-sx

As you've already found, it's possible for a question to be migrated from one of the sites to another. Thus for example TeX questions posted to StackOverflow tend to be migrated to TeX-sx as they are not 'general programming' (often having a more typesetting-related output), and as Tex-sx exists as a 'better' place to get TeX answers.