I know what an asana is, but what is a Kriya?

Kundalini Yoga is a Kriya Yoga.

Kriyas are a series of actions which include Yoga postures, breathwork, and sounds that work toward a specific outcome.
In each Kriya there are exact instructions about the times to hold the postures, where your eye focus should be and how to breathe. Similarly to baking a cake, if you added too much of one ingredient it may not be unpleasant but you may not achieve the desired result. The same can be said of Kundalini Yoga.
Teachers of this Yoga never change the formula of the technology outside the guidelines set out by Yogi Bhajan.
For me, this ensures reliability. I know that when I go to a Kundalini class that I will experience the teacher’s style of teaching, but in theory the “recipe” will stay the same. Thus you can expect a high quality class each time you attend.
One of my teachers once said to me ‘Don’t do Kundalini Yoga if you like your life! If you dedicate yourself to the Yoga of Awareness, it will change your life’ he added with a sunny smile.