Formatting
Hard drives (and other storage devices like USB keychain drives) must be formatted before data can be read and written. There are several different formatting methods that can be used, and each has its advantages and drawbacks.
Macintoshes since the era of System 8.1 use the HFS+ (Hierarchical File System Plus) format. In order to be a bootable drive for Mac OS X, a drive should be formatted HFS+ (Journaled).
(Note: an Intel Mac cannot boot from a hard drive that was initially formatted on an earlier PowerPC Mac without a GUID partition table.)
HFS+ cannot be read by Windows machines (or by a Boot Camp Windows installation on a Mac). There is commercial software, MacDrive which allows Windows to read and write HFS+.
Windows machines use both FAT32 and NTFS formats.
FAT32 (File Allocation Table 32 bit) is readable and writable by both Mac and Windows operating systems. It is a good choice for storage, such as USB keychain drives, that have to move between systems.
However, Windows artifically limits the size it can format FAT32 volumes to 32 GB (but you can format a FAT32 volume using Apple's Disk Utility - choosing the "MS-DOS" option - to any size). FAT32 also has a 4 GB limitation of the maximum size for an individual file. Normally this wouldn't be a concern, but it can get in the way of video projects and backup software, which create large single files.
NTFS is a proprietary Microsoft format, and is not an open standard. Macintoshes cannot write to NTFS formatted volumes, and reading from NTFS with a Mac is not 100% reliable. There is some software for Mac OS X that allows reading and writing from NTFS; MacFuse NTFS-3G are open source, Paragon NTFS is commercial
Once in a while you run into FAT16, mostly in very old drives or older flash memory cards. FAT16 is limited to 4 GB total volume size, and it not used for computer hard drives any more. If you have an older digital camera, do not format the memory card in a computer, always use the camera itself to format the cards.
That one will stay at my place though, where it'll only need to do anything with an iMac or a MBP.