If the disk is FAT32, you'll be able to read/write to the disk, but keep in mind FAT32 is limited to 2 GB file sizes. Also, it may have issues with Mac files you store on there, stripping out resource fork and other weirdnesses.
If the disk is NTFS, you'll only be able to read from it (out of the box.) There is a driver called NTFS-3G and MacFuse that'll let you read/write NTFS formatted disks with apparently no problems on OS X. But the driver is still in development, and as such, there's the possibility it'll hose your data.
If you intend to only use it in OS X, then I'd suggest reformatting it as HFS+, and if you still want to occasionally use it under Windows, there's some (commercial) packages that can do that (MacDisk and TransMac.)