The document defines key terms related to real number systems. It explains that the real number system consists of rational numbers like 2/5 and irrational numbers like sqrt(3). Rational numbers are those that can be represented as p/q where p and q are integers and q is not equal to 0. Irrational numbers cannot be expressed as p/q. The sets of integers, natural numbers, prime numbers, and composite numbers are also defined. Prime numbers are only divisible by 1 and themselves, while composite numbers are not prime.