The Gladius was a straight-bladed sword with two cutting edges. The blade was about 18 inches long and had a tapered point for stabbing. The hilt was made from corrugated bone, with a round pommel and usually ridges for the individual fingers.
The Gladius was primarily used for thrusting, although it could also cut and slice. It was widely used in Roman legions until late in the 2nd Century AD, when the longer spatha sword began to replace it as the standard weapon.
Many Roman soldiers would attempt to hack off enemy spearheads to disable the spears and thus negate the range advantage of the spear compared to the Gladius. This tactic was used against Celts and Macedonians, who prioritized the spear as their main weapon. However the effectiveness of this tactic is up for debate, as not all spear designs would be considered this frail.
The primary tactic used by a Roman formation was to use the Scutum to block the enemy warrior directly infront of the Roman, but use the Gladius for the enemy unit that was pinned down by the Scutum of a Roman on the right of the former Roman. https://youtu.be/f_fpOUQcAac?t=219
Gladius was a generic word for a sword in Ancient Rome, and is the root of the word Gladiator, who also used the Gladius.
Only middle and upperclass warriors could afford a Gladius; otherwise they would be light spearmen or slingers. However there are accounts of Velites using the Gladius as a sidearm, despite Velites having the reputation as being some of the poorest units in a Roman army.