Sparging is basically rinsing your mash, to extract more sugars from the grains. I would think it's always necessary in AG brewing, if only to try to maximize efficiency. There are multiple techniques, but I am only familiar with batch sparging. Here, you heat water to 170*F (the amount is determined by your recipe, somewhere in the neighborhood of 40-60% of your preboil volume). Someone else would have to fill you in on the other techniques as I have no experience with them.