Mac OS uses HFS+ by default, a file systems that is largely on par with NTFS in terms of speed and efficiency.
Macs can also read/write to FAT32, and read NTFS, only for compatibility with Windows. I'm sure they'd love to write to NTFS as well, but the file systems is poorly documented and has a number of variants that are mutually incompatible, so it is safer to only let Microsoft code write to NTFS partitions...
Um, Apple doesn't use Fat32, it uses its own file system called HFS+.
If you mean in boot camp then it is because Mac OS can read and write to it but can only read NTFS. This is because Microsoft refuses to license NTFS write abilities to anyone.
Sorry guys my mistake, I was reding about BootCamp and it said something about FAT32 on the HDD, and so mistakenly presumed thats what Macs were using...Sorry guys, cheers for the info!
Is also very interesting -- particularly the four tables starting with the one that is indexed in the link above... they do a great job of comparing the file systems.