Sutra is a word translatable into English as "book" (the name of the famous Kama Sutra translates as "The Book of Pleasure") and is generally used for scriptures of Buddhism or other Indian religions.
In Buddhism originally it was used for the written oral teaching of Sakyamuni Buddha, the founder of Buddhism. These teachings are mainly assembled in the second part of the Tripitaka which is called 'suttapitaka?' or pitaka sutra. Nowadays very often 'sutra' is taken in Buddhism in Japan and China for all kinds of Buddhist scriptures.