Query plans are merely set of steps that will be running in specific order to achieve user defined operation(s). Such ideas are widely presented in all known and unknown areas. In order to light up a fire, someone might find a piece of stick, a rock and hays, scratch the stick on the rock, make the temporature of the stick high enough to light itself or the hays. But you may just go to grocery store to buy matches or lighters and use them to burn the hays. Mapping those 2 approaches in SQL Server, those are 2 query plans.