The factors of 31 are 31,1,-1, and -31. For any number, x, the factors are the set of numbers that can be multiplied by another number to equal x.
Most numbers have positive factors other than themselves and 1. For example, the positive factors of 12 are 1, 2, 3, 4, 6, and 12. There are many special numbers, including 31, that are called "prime numbers." Any number that can only be divided evenly by itself and 1 are defined as prime numbers. The remaining whole numbers are composite numbers. The only exception is the number 1, which is considered neither composite nor prime by mathematicians.