NCBI provides a trademarked tool for sequence searching - Basic Local Alignment Sequence Tool (BLAST).
BLAST can be used to:
NCBI created a good BLAST tutorial:
The BLAST home page: http://blast.ncbi.nlm.nih.gov/
The mathematics behind this resource are fairly complicated. Please keep in mind that, depending on the specific search, the results may take a long time to appear.
The topics in this guide are:
BLASTP - Protein Sequence Search
BLASTN - Nucleotide Sequence Search
BLAST - BLAST Results