The BLAST suite

BLAST is more than just one program. In fact, there are several versions, depending on the type of sequences being aligned:

blastn compares a nucleotide query against a nucleotide sequence database.

blastp compares an amino acid query against a protein sequence database. 

blastx compares the six-frame conceptual translation products of a nucleotide query (both strands) against a protein sequence database. This translation is the simple conversion of a nucleotide string into six separate strings of aminoacids (one for each possible reading frame).

tblastn compares a protein query against a nucleotide sequence database dynamically translated in all six reading frames (both strands).

tblastx compares the six-frame translation of a nucleotide query against the six-frame dynamic translations of a nucleotide sequence database. So this program does 36 comparisons (6x6) for each comparison between the query and each of the target sequences in the database. This means that it will take the longest to complete the analysis. However, the simultaneous translation of both the query (nucleotide) and the target database (also nucleotide), allows more distantly related sequences to be identified.

(from Schneider & La Rota, 2000)‏