How to use NCBI-BLAST and read its results

BLAST (basic local alignment search tool) is an algorithm and program for comparing primary biological sequence information, such as the amino-acid sequences of proteins or the nucleotides of DNA or RNA sequences. BLAST performs “local” alignments, and this is particularly helpful when working with one or more functional domains occurring within a protein. The BLAST algorithm is tuned to find these domains or shorter stretches of sequence similarity. Moreover, the local alignment approach also means that an mRNA can be aligned with a piece of genomic DNA, as its is frequently required in genome assembly and analysis. 
BLAST works by finding regions of local similarity between sequences comparing nucleotide or protein sequences to sequence databases and it also calculates the statistical significance of matches, and displays a “expect value” or e-value that estimates how many matches would have occurred at a given score by chance, which can aid a user in judging how much confidence to have in an alignment. 
Uses of BLAST
BLAST can be used for several purposes such as identification of species, locating domains, establishing phylogeny, DNA mapping, and Sequence comparisons.
Identification of species: With the use of BLAST, we can  correctly identify a species or find homologous species. This can be useful, for example, when you are working with a DNA sequence from an unknown species.
Locating domains: When working with a protein sequence you can input it into BLAST, to locate known domains within the sequence of interest.
Establishing phylogeny: Using the results received through BLAST you can create a phylogenetic tree using the BLAST web-page. Phylogenies based on BLAST alone are less reliable than other purpose-built computational phylogenetic methods, so should only be relied upon for "first pass" phylogenetic analyses.
DNA mapping: When working with a known species, and looking to sequence a gene at an unknown location, BLAST can compare the chromosomal position of the sequence of interest, to relevant sequences in the database(s). NCBI has a "Magic-BLAST" tool built around BLAST for this purpose.[30]
Sequence Comparison: When working with genes, BLAST can locate common genes in two related species, and can be used to map annotations from one organism to another.
Therefore BLAST has proven itself to be an important tool for  studying functional and evolutionary relationships between sequences as well as help identify members of gene families.​