Most integers are composite numbers, that is, they can be written as products of two or more integers, each larger than 1, e.g. 120 = 2 x 2 x 2 x 3 x 5.
An integer larger than 1 that is divisible only by 1 and itself is called a prime number, or a prime.The simplest means of sifting out prime numbers from natural numbers is the sieve of Eratosthenes.  In a sequence of natural numbers, beginning with 2, start by striking out every second number after 2 - this eliminates all multiples of 2.  Next delete every third number after 3, then every fourth number after 4 etc.  Several numbers will be canceled more than once.  The numbers left standing are the prime numbers; those struck out are composite.