xpra or X Persistent Remote Applications is a tool which runs X clients, typically on a remote host, and directs their display to the local machine without losing any state.
It differs from standard X forwarding in that it allows disconnection and reconnection without disrupting the forwarded application. It differs from VNC and similar remote display technologies in that xpra is rootless: i.e., Applications forwarded by xpra appear on the local desktop as normal windows managed by the local window manager, rather than being all "trapped in a box together". Xpra also uses a custom protocol that is self-tuning and relatively latency-insensitive, and thus is usable over worse links than standard X.
The idea for Xpra was inspired after the original author's experience of attempting to use various NX technology based setups.