SaveOrUpdate Vs Update and Save in NHibernate

What is the difference between SaveOrUpdate and Save/Update in NHibernate. Why wouldnt you just always use SaveOrUpdate? Also, what is the point of SaveOrUpdateCopy?

Chapter 9 covers all of this better than I can:

http://nhibernate.info/doc/nh/en/index.html

But cliff notes:

Save() takes a new object without an identifier and attaches it to the session. The object will be INSERT'd.

Update() takes an existing object that has an identifier but is not in the session and attaches it to the session. The object will be UPDATE'd.

SaveOrUpdate() looks at the identifier and decides what is necessary in the above.

SaveOrUpdateCopy() is special in that say you have two objects with the same identifier -- one in the session and one not. If you try and update the one not in the session an exception is thrown normally (you are now trying to attach two objects that represent the same persistent object to the session). SaveOrUpdateCopy() copies the non-session object state to the session object state.

I'm not sure how you are going to use NH, but for a lot of cases all you need is Save(). The session is doing ALL of the work necessary to know what has to be updated and simply Flush() or a Commit() does everything you need.