SNP = variation in a single nucleotide which modifies the genome sequence. Such small difference, regardless if it occurs in coding or non-coding regions, might be responsible for a specific feature of the person who carries it (for example how a person reacts to food, his or her skin color, etc.). To be defined as a SNP, the variation must be present in an appreciable degree within a population (e.g. > 1%).
SNPs differs from alleles which represent the multiple variants of a gene due to single or multiple variations in its sequence.