The Marian Reforms were a turning point in ancient Roman military history.  They were instituted by Gaius Marius in 107 BC and can be broken down into three major reforms:

First, Marius established Rome’s first ever standing army (up until the Marian Reforms, Rome had simply enlisted its soldiers on a season-by-season basis and dissolved its army after every campaign).  Furthermore, since he was establishing Rome’s army, essentially from scratch, Marius was able to change the manner in which his new army would be organized, choosing to make the century (a group of 100 soldiers) the basic unit from which his legions were built.

Second, Marius secured the rights of the poor to enlist in the Roman army, which they hadn’t previously been permitted to do because Roman soldiers had previously been required to provide their own arms and armor, which the common people simply didn’t possess enough money to purchase.  In order to make this reform work, Marius also standardized the equipment that Roman soldiers were to use while on campaign and ensured that his new army would provide each of its soldiers with said equipment.

Third, Marius ensured that any Roman soldiers who spent enough time serving in the army would be compensated with a plot of farmland for them to retire to.  This served as the primary incentive for the common people of Rome to enlist in the Roman army, since land ownership was the best way for them to lift their way out of poverty and improve the quality of their lives.

Ultimately, the Marian Reforms had a huge impact on the Roman army and how it operated.  The Roman army was now much larger, enabling greater conquests; uniformly outfitted, enabling more complex and efficient tactics; and  more mobile, enabling faster strategic maneuvers.