A Peltast was the name of a type of troop used by the Greeks during the Persian Wars. Peltasts were lightly armed with javelins and light shields and armour.
The Peltasts would fight in combination with the phalanxes of Hoplites. At the start of a battle they'd be in a position in front of the phalanxes where they'd throw their javelins into the opposing army. Then they would retreat to safety behind their hoplites and fight as skirmishers aiding the hoplites in their fighting.