I know what the DV (digi. video) and the 24 stands for 24 fps but what is the 'pa'?

It is a format used by some cameras (like the Panasonic DVX-100) to capture 24 fps on standard minidv tapes.From, after effects cs3 help:"After Effects also supports Panasonic DVX100 24p DV camera pulldown, called 24P Advance (24Pa). This format is used by some cameras to capture 23.976 progressive-scan imagery using standard DV tapes."