As explained in the “The Three Baskets”, the sutras (Sanskrit) or suttas (Pali) are the written versions of the Buddha’s discourses and the conversations he had with the sangha. They were not written by the Buddha, but his teachings as verbalized in his “sermons” or “lectures” were written down by followers some time after his death.
There are many sutras on various subjects and of various length. It is difficult to say which ones are the most popular, because it depends on the tradition or school of Buddhism being practiced or considered. For instance, among the many sutras favored or considered of great importance in the Mahayana tradition are three; they are known as the Diamond Sutra, the Lotus Sutra, and the Heart Sutra.
Sutras are available in books and audio tapes, as well as on a variety of websites on the Internet.