Well, I figure if I start out, someone will correct.

Very simply, shantys were work songs. Now more complicated......

In the days of sail, sailors doing a long, or heavy job, would sing to keep the pace of the job going properly, and basically pass the time. The song would be led by a shantyman, which was usually an actual position on the boat.
And it would be his job to lead the boys in work and song. Alot of songs used for the task at hand, would be where the shantyman sings a line, and the rest of the sailors would refrain a line back. (musicians help with the termin. please)

for example I use Tom Lewis' 'Port in a Storm'
shantyman: All Hands to the pumps
sailors: well then tell us a story
shantyman: All hands to the pumps
sailors: well then sing us a song
shantyman: All hands to the pumps and I'll sing of the girls
All: Their a sailorman's port in a storm.


Hope that helps a bit.