Can please someone with me the difference between cascade="save-update" and cascade="persist"?I read the documentation but I do not really understand the difference.

They refer to the corresponding methods in the Session API. persist is the JPA compliant method corresponding to save. (and merge is like update)
Using one cascade mode depends one which method you use persist or save

Lets say I have a pojo named "Item" and it has a one-to-many relationship with a set named "bids"

I use in both of the xml mapping cascade="save-update" I guess that means that I can do:

If I had cascade="persist" what would be the difference?

Try and you will know :-)

For me persist is just the standard (JPA) way to say save (maybe there is some subtle differences).
If you use cascade="persiste then use session.persist() for your object instead of session.save().
I don't know if hibernate unifies the two thing internally. Maybe setting cascade="persist" and using session.save() is ok and has the same behaviour but it is not very coherent.
Maybe using persist instead of save is better because it is more standard. You can look at hibernate source code to see if save and persist are exactly the same or not.