DNA resides in the core, or nucleus, of each of the body's trillions of cells. A DNA molecule is a twisted, ladder-like stack of building blocks called nucleotides. There are four types of DNA nucleotides-adenine, cytosine, guanine, and thymine-or A, C, G, and T, for short. The order of these bases (A, C, G and T) are changed with various permutations and combinations in a sequence, and the unique sequences code for proteins.
A gene is a specific sequence of nucleotides in DNA and is the functional unit of inheritance. Different genes control the development of different characteristics in an organism.
The concept is similar to combination of alphabets to form words that further combine to form sentences. Imagine a set of books, perhaps the Harry Potter series. The entire series on the shelf is analogous to the genome. Each book can be thought of as a chromosome. Within each book are chapters, these can be thought of as genes. Lastly, the 26 letters of the alphabet are arranged to make the variation of words within the genes. The genetic code has 4 letters to make unique arrangements/sequences.