Now that you mention it I'm not sure on this.
I thought that 'Fewer Details' basically listed what was 'Tasks' back in the older version and 'More details' lists Apps (Tasks) with the resource usage and Background processes (Processes) with the resource usage like some kind of amalgamated version.
In truth I've not had to use it in anger as yet so i'll have to cross that bridge when I come to it
This is the link to the reasons and thinking to the redesign if you can be arsed to read it and it's always been one of the more widely lauded changes in Windows 8.