When you join the Active Army, your military service becomes your full-time job, and you will most likely be relocated to live on a military base. It's a 24/7 commitment for the length of your enlistment. When you join the Guard, you will be required to attend a paid drill one weekend a month and attend paid Annual Training for two weeks every summer. When needed, you can be called into full-time, Active-Duty service. But the rest of the time, you live in your local community and have more flexibility to pursue your career or education.
Also, Active Duty Army fulfills a federal mission, while the National Guard serves a unique dual mission. Either the president of the United States or your state's governor can deploy the Guard as needed, which means you might get called up to help out after a disaster in your city or state.