Sutra (सूत्र) 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 Buddhist scriptures or scriptures of other Indian religions.
The word sutra derives from the Sanskrit word for 'string'. The Prakrit/Pali form of the word is sutta.
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 Sutta Pitaka. Nowadays very often 'sutra' is taken in Buddhism in Japan, Tibet and China for all kinds of Buddhist scriptures.
In Hinduism the 'sutra' are concise descriptions of the Brahmanas, the holy scriptures.