A Sutta (Pali, Sanskrit sutra), are recorded dialogues of the Buddha designed to convey the teaching of the Buddha.
See discussion for an overview of core Pitaka.
Not really dialogues, as Wallis points out, more vehicles for preservation of the Buddha's words.
The core suttas collected into a pitaka or basket.
"As we saw earlier, the discussions that the Buddha had with others were first recorded and organized mnemonically and eventually written down on birch bark and literally sewn together as a book. While the woven book (sutta, “text”) ensures a certain degree of material preservation of the teachings, it does not ensure their vitality. For this to occur, the teachings must be further woven into the hearer-reader’s attitude toward living. This rich and complex interweaving of dialogue between the Buddha and a seeker, the physical embodiment of this as a text, and the application of this in the life of the reader constitute the real sutta."
- Wallis, Glenn. Basic Teachings of the Buddha. New York: The Modern Library, 2007.