The genetic code maps nucleotide sequences (DNA) to amino acid sequences (proteins). Every three bases (say AGT) maps to one amino acid. So you can literally read a sequence of ACGTs and decode it into a protein. A sequence that encodes a protein is called a gene.

Almost all variations that humans have in their genomes (compared to each other or a reference genome) are tiny, mostly one base differences called single nucleotide polymorphisms (SNPs). These tiny changes encode who you are. The rest of it just makes you carbon-based organism, a eukaryote, an animal, a mammal etc, just like a whole load of other organisms.