Hi Lucero, good question!
Elementary school in the US often refers to grades Kindergarten through 5th grade (Although some areas include 6th grade as elementary). In Elementary school, students generally have one teacher that teaches most if not all of the subjects. Students report to the same classroom and teacher every day. So for example, 3rd grade students have one teacher that teaches them Math, Science, Social Studies, and English all in one classroom.
Generally speaking, Middle School is often grades 6th,7th, and 8th in which students begin to prepare for their transition to high school (grades 9th-12th). In middle school, students frequently have different teachers for different subjects. For example, a student would have a different teacher for Science, English, Math, and Social Studies. Students also have to switch classrooms to go to their next class/teacher. Again, middle school is transitioning students to be more high school prepared.
These are just general descriptions and in many states you can find exceptions to these norms. I know that more elementary schools are beginning to incorporate different teachers that students visit in the day for subjects like Music and Computer Skills.
Hope this helps!