Did you try reading ANY of the other posts in this forum? They're full of people doing exactly that.

You can share files in many ways. Mac OS X supports SMB so you can directly mount PC drives on your Mac desktop.
With extra software on the PC, you can have the PC mount the Mac's drive on the Windows desktop.

Or you can use any number of IP based services to share files (http, ftp, etc.)

Both systems are able to share network resources such as printers, file servers, etc. Both can share an internet connection enabling both systems to be online simultaneously.

In short, there's many ways of doing this. People are doing it every day