The AboutAbout should cover this, but for the avoidance of doubt:
- TeX StackExchange (TeX-sx) is for questions about TeX, LaTeX, ConTeXt and related topics
- MetaMeta TeX-sx is for questions about TeX-sx itself
- Meta StackOverflow is for questions about the StackExchange network, as well as questions specific to StackOverflow
- StackOverflow 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.