TeamViewer can be used for online and offline networks. For the ONLINE part, just make sure to have a working Internet connection. Add the TeamViewer to the Exception list in Firewall and let it get its own pass and ID. Send that ID and password to the other user who wants to connect to your computer and that's it. He/she can access your computer remotely with your permission.
Note: The password changes every time TeamViewer is launched.