A hologram is an artificially created image using photons arranged in a manner to better display information...or trick people, depending on the situation at hand.
Generation One cartoon
The Autobot Hound often made use of holograms. While guarding Ravage with Mirage, Hound came up with the idea to use a hologram to trick the Decepticons and lead them into an ambush. Optimus Prime agreed, and Ravage was allowed to escape, believing he had learned of a military base with large amounts of rocket fuel. Megatron, however, saw through Ravage's escape, sending the Decepticons to the real base, while he went with a group of shoddy Decepticons to rub it in Prime's face. The hologram of the base worked just fine, though...
Shockwave was using a holographic projector to train with Sentinel drones when the Combaticons arrived and attacked them. Later, Shockwave and Starscream used the holographic projector to create various images of hostiles. This successfully distracted the Combaticons for some time, until Swindle discovered that their attackers were holograms. The Combaticons aren't really smart. That's why they let Onslaught do all the thinking.
IDW comics continuity
Recognizing that the Autobots' lack of drivers would attract attention, they used Holomatter avatars to avoid detection. However, security measures can be devised specifically to detect holomatter, showing that organizations such as the Machination already anticipated Cybertronian interference.
Beast Wars/Beast Machines
By the Beast Era, holograms were in more wide scale use, including such applications as entertainment and military use. The Axalon and the Darksyde both made extensive use of hologram technology to run simulations and observe events in real time. On Cybertron, there were holo-chips which allowed Transformers to interface with them and see things virtually—some of which were not always virtuous, if you get our meaning. .
The Autobots used holographic drivers while on Earth. At one point, the US President's image is used by Six-Speed in order to avoid trouble with the police. The holograms are translucent, but human viewers don't seem to notice.
Live-action film continuity
After introducing the Autobots, Optimus Prime showed Sam Witwicky and Mikaela Banes a hologram of the devastation Cybertron suffered during the war with Megatron. The Decepticons also made use of holograms, using a single, mustached male as a display to avoid attracting attention. Oddly enough, the Decepticon holograms appear to have solid machinery underneath them, but when activated they appear to be a projection with nothing under the surface.
During the Shanghai battle, Arcee uses three hologram girls for each of her components.
In Transformers Animated, Prowl will sometimes project a hologram of a mustached police officer riding him while in vehicle mode, as to remain inconspicuous. Nothing says "conspicuous" like a motorcycle without a rider. He's also fond of creating false images of himself, the better to ambush enemies who attack the wrong Prowl.