Sutra is basically a type of religious literature that is found in religious countries. The term Sutra is known as the doctrines of any religion such Hinduism, Buddhism, Jainism, etc. According to religious tradition, Sutra is believed to record important religious practices as well as teachings of important figures in particular religion. Sutra are also known by the term thread which is believed to hold things together or basically it holds all the teachings as well religious practices together. The main purpose of sutra is to record the teachings as well as the religious practices and in order to pass it on later generation so that they can follow the sutra and walk on the path of their previous generation paths.
Sutra in Buddhism
In Buddhism, sutra is basically referred only to the sermons of the Shakyamuni Buddha. In Buddhism, the tradition of written sutra came to exist after the success of First Buddhist Council. According to Buddhist history, Ven. Ananda attained Enlightenment a day before the First Buddhist Council and gave sermon from his memory. And from that sermon, a part of Tripitaka i.e. Sutra-pitaka was collected. In First Buddhist council, Ven. Mahakasyapa and Ven. Upali and other senior Buddhist disciples of Buddha gave sermon and took part in the compilation of various sutras that contains the teachings of Gautama Buddha.
After the First Buddhist Council, two oldest Buddhist traditions were introduced in the world i.e. Theravada Buddhism and Mahayana Buddhism. Even though the sutra containing the basic teachings of Buddha are mostly the same, but the sutras that contains the religious practices are slightly different in both tradition since these two tradition have different principles of attaining final goal of Buddhism i.e. Nirvana. The Mahayana sutras were mostly written and compiled 5 centuries after the death of Lord Buddha. Most of the Mahayana sutra were written in the way that Ven. Ananda preached in the first Buddhist Council.
Some examples of Buddhist Sutras
Now in the present time, there are around uncountable number of Buddhist sutras that are written and compiled in different tradition. Some of the most popular Buddhist sutra that are even known to beginner Buddhist monks are Lotus sutra, Heart sutra, diamond sutra, avatamsaka sutra, prajnaparamita sutra, vimalakirti sutra, etc.
You can find number of Buddhist sutra that are quite important for the training of junior Buddhist monks. In many Buddhist community or Sangha, many young Buddhist monks are taught basic teachings of Buddha through various sutras. So Sutra can be considered very important.