The Hundred Year War marks the transition from Medieval wars to modern warfare. Some of the revolutionary innovations that set the two apart are:
1) Infantry revolution: Medieval warfare was mostly conducted by lords and their soldiers who served mostly as cavalry, were captured rather than killed, were heavily armored and relied on muscle power. In the Hundred Year war, the soldiers were "professional soldiers" who were paid 3 pence/day, were drawn from the general population, mostly fought on foot (infantry) and fought to kill and not capture. This success of infantry can also be attributed to the new six-foot yew longbow which enabled archers to penetrate enemy armor, which was not possible with previously used four-foot Welsh elm bows.
2) Artillery revolution: The advent of cheap gunpowder and cannons made siege more effective. During medieval times, defense rather than offense was a better approach. However cheaper and better cannons ensured that fortification would be of less value and that forts or parts of the city could be knocked down by cannon fire. This shifted the momentum towards the offensive.
These two changes revolutionized medieval warfare and propelled us into modern warfare.