Remmina is a remote desktop client that support multiple network protocols (RDP, VNC, NX, XDMCP and SSH) in a nice and very user friendly GUI.
In its basic usage, you only need to know the protocol, the server (domain or IP address), username and password.
For example, this shows how to connect to a local Linux machine through NX:
This works perfectly if you are in the same LAN or your machine has a public IP address.
It couldn't be any simpler than that!
NB: In Ubuntu, Remmina doesn't come with the NX plugin installed, in that case you need to:
apt-get install remmina-plugin-nx
If your machine does NOT have a public IP address though, which is the most common scenario, and you are not in the same LAN, you might need to set up an SSH tunnel.
As shown here above, you only need to enter the IP/hostname of the server with SSH server setup, a username and some SSH credentials.