Skirmishers are infantry or cavalry soldiers stationed ahead or alongside of a larger body of friendly troops. They are usually placed in a skirmish line to either harass enemy troops or to protect their own troops from similar attacks by the enemy. Skirmishers are generally lightly armoured for increased battlefield mobility and are usually armed with ranged weapons to attack the enemy from a distance.
Once the preliminary skirmishing was done, the skirmishers would participate during the main battle by shooting into the enemy formation, or could participate in melée combat with daggers or short swords. Alternatively, they could also act as ammunition bearers or stretcher-bearers.
Due to their mobility, skirmishers were also valuable for reconnaissance, especially in wooded or urban areas. During the gunpowder era, a skirmish line could be used to discover the extent of the enemy front line.
In classical Greece skirmishers had low status. For example, Herodotus, in is account of the Battle of Plataea of 479 BCE, mentions that the Spartans fielded 35000 light armed hellots to 5000 hoplites yet there is no mention of them in his account of the fighting. Often Greek historians ignored them altogether. It was far cheaper to equip oneself as light armed as opposed to a fully armed hoplite - indeed it was not uncommon for light armed to go into battle equipped with stones. Hence the low status of skirmishers reflected the low status of the poorer sections of society who made up skirmishers. On top of that however was the way that hit and run contradicted the Greek ideal of heroism - in Plato the skirmisher is given a voice to advocate "flight without shame" but only in order to denounce it as an inversion of decent values. Nevertheless, skirmishers chalked up quite a few significant victories such as the Athenian defeat at the hands of the Aetolian javelin men in 426 BCE and, in the same war, the Athenian victory of Sphacteria.
Celts did not favor ranged weapons. The exception were the British who used the sling extensively but in siege warfare not as skirmishers. The Celtic contempt towards skirmishers was to cost them dearly during the Gallic Invasion of Greece of 279 BCE where the Gallic warriors found them helpless in the face of Aetolian skirmishing tactics.
In the Punic Wars, despite the fact that the Roman and Carthaginian armies were organized in very different ways in other respects, skimishers had the same role in both - to screen the armies