Mac OS X uses the SMB (Server Message Block) protocol for file sharing with Windows users, as well as Unix/Linux users. This is the same protocol that Windows uses for network file and printer sharing, but Microsoft calls it Microsoft Windows Network.
OS X 10.6 supports two methods of sharing files using SMB: Guest Sharing and User Account Sharing. Guest Sharing allows you to specify the folders you wish to share. You can also control the rights a guest has for each shared folder; the options are Read Only, Read and Write, and Write Only (Drop Box). You can't control who can access the folders, though. Any individual on your local network can access shared folders as a guest.
With the User Account Sharing method, you log in to your Mac from a Windows computer with your Mac username and password. Once you're logged in, all of the files and folders you would normally have access to on your Mac will be available.
The User Account Sharing method may seem to be the most obvious choice when you want to access your Mac files from a PC, but there's a slight possibility that your username and password could be left behind and accessible on the PC. So for most users, I recommend using Guest Sharing, because it allows you to specify the folder(s) you want to share and leaves everything else inaccessible.
One Important Note About SMB File Sharing
If you have User Account Sharing turned off (the default), anyone who attempts to log in to your Mac from a Windows computer will be rejected, even if they supply a correct username and password. With User Account Sharing turned off, only guests are allowed access to shared folders.