Hi 
It is the files on the drive (or rather, partition) that get fragmented. Not the partition itself.
Ntfs tries to cramp all the files onto the beginning of the drive. When you add things to a file then it can no longer fill the space comfortably so a chunk of the file needs to be written further along. Once you have added things to quite a few files there is a bit of a mess which Ntfs then has trouble in tracking and the read/write head bounces backwards and frowards all around the partition collect all the parts of the file.
Ext2, 3 and 4 all leave comfortable gaps after the ends of files and if the file still wont fit into the extra space that has been reserved for it then it just tries to write the entire file elsewhere or maybe (rarely) shift a few other files around to make space. Files do occasionally get fragmented but not quite as often and usually only when some fool has nearly filled their hard-drive.
Then i get a bit lost because although Ext2, 3 and 4 carefully avoid fragmentation they use the "inode" system for keeping track so that even if files do get fragmented the OS doesn't suffer. So why bother to avoid fragmentation? What is inode?
The clincher is that most Windows defraggers (or at least the official MS ones) can't defrag system files (or at least not well and not easily), critically the "virtual memory" (which gets called "swap" in Gnu&Linux).
Since virtual memory was so crucial to a system one might have expected it to be set to avoid letting it fragment ever. On Windows systems it's reasonably easy but very fiddly to set virtual memory to a fixed amount and that significantly stops any further slow-down. You still get some due to the registry kludge and other mis-management of key resources but the major difference seems to happen when you defrag and fix the virtual memory.
On Gnu&Linux systems the Swap is often given it's own 'drive' (really a partition on a drive) so that it doesn't have a chance of getting fragmented, ever. Again this can be done in Windows but it's never the default. Also Swap/Virtual-memory in Gnu&Linux can be set to be a file instead of a partition and that might be easier for people who only need a swap in the unlikely event they hibernate. Again it's weird that Gnu&Linux takes an extra unneeded precaution against something it 'shouldn't' ever suffer from but the one platform that does suffer from it doesn't do anything to protect against it.
It's just one example that makes me wonder if Windows is deliberately set-up in order to force users to buy new hardware and a whole new system every couple of years.
Regards from
Tom 