Beacons were a system of near-instantaneous communication across the Protheans' galaxy-wide empire. They were made to interface with a person's brain, rather than through audio or video.
Commander Shepard's 'vision', triggered by accidentally accessing a beacon, was actually a warning about the Reapers sent by the researchers on Ilos, in an attempt to reach any survivors and give them a message of hope. The message sent through them was coded specifically so that only Prothean minds could understand it. The Protheans hoped this would prevent any remaining Reapers from accidentally finding out that Ilos had survived. However, at that point, the Protheans did not fully understand the process of indoctrination, and they did not realise the message could be received and eventually understood by an organic agent of the machines.
This precaution -- and the damage to the Eden Prime beacon -- meant that Shepard received the information as a series of random, vivid images. These caused an initial fifteen hours of unconsciousness, and left Shepard with recurring nightmares. Comprehending the message as it was meant to be seen required the Cipher, which enabled both the commander and Saren Arterius to comprehend the images and archetypes in the beacon's message.
The Eden Prime beacon was badly damaged, reportedly in Sovereign's attack on Eden Prime. Not only did it respond without warning when approached, the 'vision' it held was confused and incomplete. It exploded after being used; it has been suggested that the strain of transmitting it to Saren and then Shepard in a short space of time caused a system overload. Liara T'Soni comments that Shepard is exceptionally strong-willed, making her feel exhausted after touching the commander's mind because humans instinctively resist the joining process. Therefore the system overload could have been caused by Shepard instinctively fighting the beacon's vision, instead of welcoming it as Saren did.
The beacon in Saren's lab on Virmire was intact and had an interface for users to access it. When using the beacon, Shepard was able to cope more easily when the vision was transmitted. It isn't clear whether Saren found this intact beacon on Virmire or had it brought there from another planet. The Virmire beacon was presumably lost in the nuclear explosion that destroyed Saren's base.
The video terminal's vision on Joab is partly identical to the threat about Sovereign and the Reapers but instead of a Reaper it shows a pair of Collectors in the end.
Another beacon was discovered hidden on Thessia, the asari homeworld, the source of the asari's advanced culture. During the Reaper invasion, Shepard discovered the beacon and activated it, releasing a Prothean VI named Vendetta. It provided data that suggested that the repeating patterns of galactic evolution and development were not, in fact, caused by the Reapers, and that the Reapers are merely "servants" of that pattern. But before it could give the data on the final Crucible piece, Kai Leng appeared and stole the data.