A gene has alternative forms (alleles) that arise by small changes in its sequence. Only one of these variants is found at the correct place on the chromosome. SNPs are a specific case of variants in which the difference regards only one nucleotide.
The genotype is the set of pairs of genes, one for each chromosome inherited from the mother and from the father, which are responsible for a particular trait. The phenotype is the physical or physiological expression of that trait.