Basic Local Alignment Search Tool is a type of search used by biologists to search for alignments of either protein sequences or nucleotide sequences with known seqences stored in the National Center For Biotechnology Information's databases. Such a search is invaluable to molecular geneticists because it will help to identify a sequence as a previously identified gene. To an evolutionary biologist, this is useful because showing that large sections of an organism's genome aligns with another organism's genome is strong evidence that both organisms shared a recent common ancestor.

Altschul, S.F., Gish, W., Miller, W., Myers, E.W. & Lipman, D.J. (1990) Basic local alignment search tool. Journal of Molecular Biology 215: 403-410.

More specifically, BLAST is a "local" alignment search because it optimizes the alignment between two sequences for the most homologous regions, as opposed to trying to align the whole sequences in the most optimal way. Because of the way DNA and protein sequences evolve, this is often the best way to find evolutionarily relevant similarity between two sequences.

A convenient website to BLAST from is http://www.ncbi.nlm.nih.gov/BLAST/