The people/time period I'm talking about is the: Greeks, Romans, Macedonians (At the height of their power), and Napoleon. Obviously there were no radios, so how did the general tell his troops where to go during a battle?
The Commander - 'General' is a term not used much until the late middle ages - would have a fairly large staff with him. He would usually issue verbal orders to one of these staff members and they would go to the subordinate commander and relay the message. Until the advent of the radio, this was the standard practice used by commanders to issue orders etc.
Aztec battlefield communication started from the basic level, with military coordination of smaller number of troops based around a "captain." These were from the calpulli system of organization, which divided cities into neighborhoods, each with its own telpochcalli (youth house) acting as both school and military academy. From the the calpultin, groups of several hundred men familiar with each other and having trained together could quickly be mustered. Leading them would be experienced older men, with unit leaders bearing a cauchpantli, a back-flag. You can see some of the more elaborate designs in this image form the Codex Mendoza.
Above that level of several hundred men, there would be larger banners carried like more traditional flags. Hassig, in Aztec Warfare identifies 4 such banners and suggests this represented a 4-way division of an army, which would fit with Aztec cosmological view as well as simple practicality.
Banners and flags, however, serve best as rallying points and identification. Certainly a soldier could just follow his unit's flag and the captain could follow cues from the larger banners, but top-down orders were more audio-visual. The most important methods of signalling were through the use of drums and conch-horns. One of the most famous depictions of the Texococan leader, Nezahualcoyotl, actually shows him carrying a signal drum on his back.
hi! you'll find additional information in the FAQ (link on sidebar):
and more on ship-to-ship comms under Communications
One of the most common ways to communicate during the Napoleonic period was through Aide-de-camp and a small staff of educated officers who acted as runners for a general. Often times, these runners had a bright uniform which ensured that they would be a visual cue to subordinates that there was a command coming. A prominent example of the Napoleonic period's ADC were Berthier's private staff. Berthier had the unenviable task of translating Napoleon's commands into an operational reality. His couriers were distinguished by a bright hussar-style uniform (see here- http://theminiaturespage.com/boards/msg.mv?id=277226 ) and their campaign duties amounted to furtively scurrying around the battlefield.
Hellenistic commanders penchant for imitating Alexander often led to a lack of battlefield command unless they decided to place themselves behind their own lines. Most commanders, like Eumenes, Antigonus I, Ptolemy IV, and Antiochus III the Great stationed themselves with the hard-charging shock cavalry. Moreover, while the Macedonian phalanx was difficult to manage tactically considering the weight of kit and its dense formation, it usually operated by moving forward against the enemy which was usually another phalanx until the arrival of the Romans. Once the phalanx was arrayed it didn't need much exhortation for its members to understand where the enemy was. Command and maneuver was most often done by the commanding general leading the cavalry, just as Alexander had done for three of his four great victories.
http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0234%3Abook%3D5%3Achapter%3D82
