In a globally connected world, it's harder than ever to "be there" without actually being there. Voice Over Internet Protocol (VOIP) and video over internet allowed families to connect from far away over technologies like FaceTime and Skype (among many).
Telepresence is a set of technologies intending to increase that sense of being present in a remote (or even completely unreal) environment. As you can imagine, such a broad goal has created some very different ideas.
Immersive telepresence technologies connect one or more people in a completely simulated world. Imagine any massively multiplayer online (MMO) game you've ever seen. Or Minecraft for examples of people working together toward a common goal in a completely virtual world.
You may have seen augmented reality technologies that work by mapping elements in a virtual world to a position in the real world. A see-through screen or camera captures live and virtual elements putting them together.
iRobot, the company that gave you robotic vacuums and mops that navigate your home is also working on solutions to allow you to roam an open space remotely. Called the Ava 500, the robot is what you might currently expect from teleconference technology - picture and sound. Where this innovates is that the camera, screen, speaker, and microphone are mounted on a simple robot. This allows the visitor to roam, squeeze in for a better look, and do many other things that watching from a fixed camera angle simply doesn't allow.
Long a goal from science fiction movies and television, Cisco is featuring stage holograms very much like you've seen on television.