Hi,
There is no one implementation. Every system/application has specificic requirements and needs specific permission system.
There is a question on StackOverflow which has great answer. You definitelly should take a look at it:
http://stackoverflow.com/questions/3177361/modelling-a-permissions-system[
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Most popular is ACL (Access Control Lists). I'm using this model (with some modifications) in business application I'm currently developing.
I hope it help you.