Implementing access with oAuth or other

[Prift2004]

I'm trying to think of a way to open a website and part of its database to other third party websites, similar to how Twitter lets webapps connect to its database to retrieve data and possibly store data. My initial research led me to oAuth (or is it openID?). What I need to do is let the third party websites login to a user's account on the website, but only read and write data that belongs to them and not be able to touch data that belong to other third party websites?I'm a bit fuzzy of the details of oAuth. Do I need to create some sort of an API or is oAuth the only thing I need to implement? As you can see, I'm not sure on how to do this so any explanations from those who are more experts would help.If I implement oAuth, will other third party websites be able to access data from the website? Can I let other third party websites register the user to the main website without the user having to visit the main website? Does anyone think this is a bad idea? yes/no and why?and how does oAuth help me make sure that a third party websites does not read/write/change user data related to another third party website. I'm sure this is easier than what I think, but I'm new to oAuth so the picture isn't clear yet.