[galaxy-user] megablast

I have a large set of short reads (from human) that I'm trying to analyze with galaxy. Specifically I want to map my nucleotide sequences to the human genome. I found the "short read analysis" section of galaxy. However, I can't seem to find any information on how to interpret the output from megablast. Could someone tell me what the numbers in the different columns represent? And when chosing target databases, what is "nt" and "wgs"? Thanks in advance, /Ali