The reforms typically attributed to Caius Marius generally mark the border between reference to the Republican army and the Late Republican army.  Though Marius receive the broad majority of the credit—indeed, the transformation is called the “Marian Reforms”—it should be understood that the majority of these reforms were already in use long before Marius’ rise to prominence; rather, Marius is used as the catalyst between the army of the Middle Republic and that of the Late.  The greatest contribution made by Marius to the Roman army was to reduce the size of the baggage train which accompanied legions on campaign by requiring legionaries to carry a broad majority of their equipment.

Regardless, the greatest distinctions between the function of the army during the Second Punic War (Middle Republic) and the Gallic and Civil Wars (Late Republic) were the prominent use of cohortes as a building-block tactical formation.  As a central unit of the army, the cohort itself was the birth child of Roman experiences in Spain, namely against Numantia (154-151 BC, 143-133 BC).  In the mountains of the Iberian Peninsula, it was discovered that manipuli were simply inadequate for the sort of long-term deployments necessary in conquering those lands, and legions were far too unwieldy.  Rather like the shift in the modern U.S. Army from regiment-centered fighting formations to brigade-centered ones, the Roman Army moved towards the more convenient and obvious sort of fighting units.  Larger  and better organized than manipuli, but not nearly the size of an entire legion, cohortes were capable of both sustained autonomous actions as well as actions as an integral part of the legion.  By the time of Marius and Sulla, the change from manipuli to cohortes was complete, and though the manipuli remained an important part of convention in the Army (such as the titles of the individual centuries), they were no longer the central fighting unit.

Along with the shift to cohort-centered formations, Marius is additionally credited with the creation of an all-volunteer army rewarded by stipends and land; it is in fact a myth.  Although legionaries were indeed recruited from volunteers and repaid with stipends and even lands, the conscription of soldiers remained a common practice—indeed, the broad majority of soldiers in the army of the Late Republic remained conscripts.