Known in Mexico as Patadas Voladoras, this is the standard two foot kick done by good guys the world over.
Standard dropkicks are down to the chest, but pretty much anything that can be kicked can be dropkicked. A Dropkick to the Knee is the usual set up to La Casita and other ground based cradles and submissions. Multiple dropkicks to the head are popular in trios matches.
Dropkicks from the top rope are called Flying Dropkick' or Missile Dropkick.