Xpra is "screen for X": it allows you to run X programs, usually on a remote
host, direct their display to your local machine, and then to disconnect from
these programs and reconnect from the same or another machine, without losing
any state. It gives you remote access to individual applications.
Xpra is "rootless" or "seamless": programs you run under it show up on your
desktop as regular programs, managed by your regular window manager.
Sessions can be accessed over SSH, or password protected over plain TCP sockets.
Xpra is usable over reasonably slow links and does its best to adapt to changing
network bandwidth constraints.