This page deals with some policies that apply widely across the Wikimedia projects.
Sources of authority
The source of authority on Wikimedia policy is various, see also Wikipedia power structure.
Policy evolution
Policies have evolved on various Wikimedia projects (particularly the original English Wikipedia project, now at en.wikipedia.org), when common problems were solved by collectively deciding on suitable procedures to avoid them.
New wikis are constantly being created. What tends to happen with a new wiki is that either (i) someone will fork a copy of another wiki's policies to the new wiki, or (ii) the new wiki will build up its own policies from scratch, or (iii) there will be a combination of (i) and (ii).
Over time, these policies have tended to diverge, but end up largely replicating each other (e.g. the NPOV policy). Consequently, a lot of time is wasted when users unwittingly debate the same policies over-and-over on different wikis, unaware that it has all been done before. By placing all policy in a centralized location, this effect can be minimized.
Enforcement
All users can enforce policy. Certain procedures can only be implemented by an administrator, but even the anonymous user has sufficient power to implement most policies. Policy that can be implemented by any user should be encouraged, as the current administrative system can be somewhat impractical on less popular wikis.
Repeated vandalism is a major breach of policy; e.g. the unexplained blanking of content, or spamming a page with inappropriate content, especially linkspamming. These actions will be grounds to ban a user accounts and/or IP address.
Generally, there are no "punishments" for minor breaches of policy, which are addressed when another user edits an offending page to make it compliant.
Current policies
See Meta:Policies and guidelines for a reasoned list of policies and guidelines hosted on Meta-Wiki.