The Stack Exchange sites are run by a for-profit company. Their ultimate goal is to make money by providing a service on the internet. The service is host question-&-answer sites.
The level of each site depends to some degree on decisions made by the user base. Stack Overflow (the original model) accepts questions starting from a very low, but not trivial level.
Physics.SE is pitched just a little higher than that: we discourage basic pedagogical questions (things that look like "homework") in favor of a conceptual approach to the foundations of our discipline. Note that there
is also used to be Theoretical Physics which is was targeted at "research level" theoretical and mathematical physics but was deemed to be diying and merged with this site, look for research-level to find that and related content.
For other sites in the network, see the bottom of the page and area51 (the place where user input defines and launches new sites, and where you can find all the beta sites).