Chromosomes, DNA & Genes
The body is made up of over a trillion cells and includes different types of cells such as skin, liver and blood cells. Typically, all of the cells in our bodies, except for our mature red blood cells, contain 23 pairs of chromosomes. We inherit one of each chromosome from our biological mother and one from our biological father. Our chromosomes contain the instructions for the development and functioning of all our cells.
Chromosomes are composed of long strands of DNA tightly wrapped around proteins. Each cell in our body contains approximately 1.8 meters (6 feet) of DNA in total, although each strand is less than one millionth of a centimeter thick. 
Our DNA contains around 25,000 genes. A gene is a piece of DNA that contains the instructions to make a specific protein. Proteins are important because they tell each of our cells what to do and when to do it. Some proteins give our body shape and structure while others help cells to do things like digest our food or carry oxygen in our blood.
In most cases, we inherit one of each gene from our mother and one from our father. Each gene has a different sequence made up for four letters, A,T,C,G, called bases. People can have variations in their gene sequences. These variations make each of us unique. For example, people can have a rearranged sequence, pieces missing in their sequence, called deletions, or pieces added called insertions.
SNP (Single Nucleotide Polymorphism).
SNPS and other gene variations are what make each of us different. For example, gene variations can determine what color of hair we have or the size of our feet. Some variations can also make it more likely for somebody to get a certain disease such as diabetes. Variations in certain genes can also influence our weight.
Bianconi E et al. An estimation of the number of cells in the human body. Ann Hum Biol. 2013 Nov-Dec;40(6):463-71.
 University of Leicester. DNA, Genes & Chromosomes. http://www2.le.ac.uk/departments/genetics/vgec/schoolscolleges/topics/dna-genes-chromosomes