The biggest difference is that the Army Reserve is a federal reserve component and therefore are only used for federal missions. The Army National Guard is a state reserve component and can be used for state and federal missions. The Governor of each state is the commander in chief of the National Guard in their state. The President of the United States can, with the governor's consent, call National Guard units to federal active service for National Emergencies, or overseas deployments. Individual states have no control over Army Reserve units, and therefore units can be transferred from state to state. One of the benefits of the National Guard, is that since they are run by the state and federal government, many National Guard soldiers get College Tuition Assistance from their state funds as well as the Federal government funds.
Both Guard and Reserve soldiers drill one weekend per month and two weeks each year, so in that sense, they're identical. The main difference is in the upper levels of leadership.