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In the most basic terms, genome is the genetic material of an organism. It is what tells the organism what to be and how to be. The genome is coded in DNA, or in RNA for many types of viruses. The term, genome, was coined in the 1920, as a portmanteau of gene and chromosome.
Chromosomes contain smaller units of genetic material called DNA. DNA is a sequence of letters that spell out the genetic code. The DNA is organized into words and sentences called genes. Humans have approximately 20,000 genes and each one influences a part of development. These genes usually come in pairs.