Real numbers continue to be our bread and butter in Algebra II. They include the integers (positive, negative, and zero), as well as the rational and irrational numbers. Imagine those family get-togethers; they're all so different, it's amazing they can get along at all, much less work together as a complete number system.
They do work together, though. We can add, subtract, multiply, and divide any combination of real numbers together as we please, and it all makes sense. Except division by 0: that does not work out, and it does not make sense. In a "mathematics is broken forever" kind of way.
All of this sounds familiar, right? If not, consider the possibility that someone has snuck into your brain and has stolen some of your memories. If that seems a bit far-fetched, go take a refresher, courtesy of Shmoop, instead.