A resident has a much more predictable working life - works just one location, and has a fixed base roster so will know what turn they're booked to work on a given day months in advance. That's not to say that they can't be asked to work a rest day, cover a different turn one day or even go to a different location if they sign it...they're not compelled to do anything other than what's shown on the base roster though.
A general purpose relief man (which I think most are nowadays) will usually cover more than one location, and generally get messed about a lot more. They're still guaranteed their rest days and Sundays (if not shown to work them on the base roster) off if they want them, but can be sent anywhere and for any turn (within the agreed turn length) on the other days. That's a bit of a simplification, of course, and there are rest day relief men in some areas too. There's a flexibility premium, which makes up for the lack of certainty and stability to a certain extent...that's based on the distance (or time) to get to the most distant location covered from the 'home' location. It's not for everyone though!
