What file system are memory cards formatted with?
They're formatted with FAT16 or FAT32 (FAT32 is required for card sizes >2GB), and have a fairly specific (though simple) directory structure something like this:
ROOT --- DCIM -+- ###ABCDE | +- ###ABCDE | ...
###is from 100-999, and need not be consecutive. ABCDE is free text.
This structure won't be recreated if you format on a computer, but an in-camera format will usually construct it (or it will be built as required on a blank card).
There are similar standards for file names, and it's all specced out in detail by the Design Rule for Camera File System, which you can read in full here (PDF):