Techniques are the words of Aikido.
My training consists, largely, of practicing techniques over and over. Techniques are structured movements, patterned responses to predetermined attacks. They have identifiable forms that, while differing in small details, are essentially the same across Aikido styles. In and of themselves, techniques are mechanical vehicles for learning and internalizing the concepts that underly Aikido. I learn connection, congruent motion, correct distance, timing, extension of Ki while moving etc.
Randori is the literature of Aikido.
During randori the motions of uke and nage are removed from the structured dance of technique practice and enter into the realm of spontaneity. My uke is free to move unfettered by the constraints that govern technique practice and, likewise, my movement is equally unrestrained.
Early in my training, randori consisted of repeated attack/throw sequences, not far removed from the practice of individual technique; the obvious difference being that the attacks and defensive moves were not predetermined. Of late the formal waza has begun to disappear from my randori; it's being replaced by something else that I can't yet define or adequately describe. My partners still fall, but it's as though the throws are hidden in the motion. The whole exercise becomes an unbroken string of movement punctuated by an occasional slap of a hand on the mat as uke falls.
The following video clip of a 'grabs only' randori illustrates the point.
(Original blog post may be found here