Think of lashing as a form of weaving poles together to connect poles together. Depending on what you need the poles to do depends on the weave, exact lashing you are use. For example: you use a tripod lashing when you want 3 poles to stand up like a tripod over say a campfire, or square lashing when two poles cross each other like a cross, a diagonal lashing when two poles cross forming an “x”, sheer lashing is when you want to weave poles together to create say a raft, and round lashing when you want to join two poles together to make one longer pole like maybe a flag pole.
For more details about lashing take a look at http://en.wikipedia.org/wiki/Lashing_(ropework).
These images and other great info about knot tying and lashing can be found at http://www.bsatroop542.org/Knots.htm