Quick Casts can be played at any time and as a reaction to another Cast or Skill.
Quick Casts create a chain of events and will resolve with the last card activated going first. (i.e., you activate De-Rank on your turn. In response your opponent activates Counter Cast. In this instance Counter Casts effect triggers first and negates and destroys your De-Rank before it can resolve.)
Only a Quick Cast or Quick Skill can be Chained to a Quick Cast.