SNPs ("snips") are single base-pair (one rung of the DNA ladder) variations that occur in the coding region of chromosomes.
If you take two people's DNA and line them up, the variations that you see between the two strands are SNPs. It's what makes you different from me.
SNPs are what organizations and companies are currently looking for since the Human Genome Project has mapped most of the 3GB (three gigabasepairs, or three billion base-pairs) that constitute our genome. By finding the exact genetic variations between individuals, one can assume that cures for genetic diseases can be derived.