that means that you pick what day and time that you want to work. Generally per diem offers more pay cuz there are no benefits. Some places have a regular per diem staff and some places have per diem to be floats...check it out where you want to go!
My unit works like the others stated: a few dollars more per hour, no benefits, you pick which shifts you want to work but you are the first to get cancelled for low census. I'm not sure if there is a minimum requirement on my unit, but I do know that some hospitals require one weekend a month and sometimes one major and one minor holiday or something to that effect. I would like to have a per diem job in addition to my full time job, but the weekend and holiday requirements for per diem positions around here would mean I'd hardly have any weekends or holidays off!