Holograms are a technology present in the Deus Ex series that is used to construct three dimensional images out of light. They are often used for communication.
Holographic terminals are in use in several games, there are also occasional holographic decorations.
The image quality of holograms vary across the games. Holograms in Deus Ex and Deus Ex: Human Revolution are similar to in game graphics with only a slight transparency betraying their holographic nature. Simple communication hologram in Deus Ex: Invisible War are tinted blue with lines strobing though the images. This is likely a design choice to attract your attention to the hologram projecting device. Holograms projected from NG Resonance kiosks tend to have a slightly higher quality. Eliza Cassan is the highest quality hologram known, being virtually indistinguishable from a real person in appearance.
Hologram communication originally seems rare and expensive, only being seen in use by the wealthy in Human Revolution. This trend seem to continue in Deus Ex but they have become more common by Invisible War, potentially replacing normal phones in some houses. These Holocomms are present in many homes and businesses.
In the games there are several human shaped holograms that are representations of AIs. These are NG Resonance kiosks, Eliza Cassan, and Ava Johnson. In the case of Eliza her hologram is good enough to be mistaken for a real person, even at close range. It is possible that Eliza Cassan's hologram has a physical component, as Pritchard mentions a "holo processing cloud".
Morpheus also has a crude holographic display, although not one that depicts a human shape.