An SSH client gives you command-line access to a Linux server.
Mac: SSH is already built in. Just open a Terminal window and type "ssh" to use it.

VNC Viewer

A VNC viewer allows you to access your remote desktop on a Linux server.
WindowsRealVNC Viewer for Windows
MacRealVNC Viewer for Mac

WARNING: Do not install "VNC Server". The "VNC Viewer" is all you need.