Hayom Yom: Sivan 15

At first the Alter Rebbe would deliver extremely brief expositions that aroused a storm within the listener's heart, and greatly inflamed his emotions. These were called d'rachim, "pathways."

Later his talks were a little more extended, and these were called igrot, "missives."

In the next stage of development, his talks were termed torot, "teachings," and these formed the basis for the maamarim in Torah Or and Likutei Torah.

Later came somewhat longer teachings called k'tavim, "writings." These were expositions with comparatively broad intellectual elaboration.