The Upanishads are part of the Vedas, the oldest scriptures of the world. There are four Vedas, namely the Rig, Sama, Yajur and Atharva Veda. Each Veda is further divided into four parts which are called Samhita, Brahmana, Aranyaka and Upanishad. The first three parts of each Veda mainly deal with rituals. The Samhita part consists mostly of prayers to different deities and is used for sacrifices. The Brahmanas deal with how the sacrifices have to be performed, and the Aranyakas give the philosophy of the sacrifice. The Upanishads deal with the philosophy of the Vedas.
One can approach the Vedas with a traditional or a modern understanding. The tradition says that the Vedas were revealed by God himself to humans at the beginning of creation, therefore there is no date on them. Modern scholars have different opinions about when they were written, but they have no concept of who wrote them.
The word Veda means knowledge. The Vedas are the books of knowledge. In that sense they are not sectarian, because they are not related to a particular group of people or faith; rather, they are the manuals for human beings. They show how one should lead one’s life on earth and attain perfection.
Another name for the Vedas is sruti, because they are heard from the teachers and “sru” means to hear. If someone wanted to study the Vedas, traditionally he had to go to a teacher and hear them from him. Vedic knowledge is divided into two categories, known as karma-kanda and jnana-kanda. The former stresses the rituals and sacrifices for attaining material gain. The latter deals with the ultimate goal of life—realizing the Absolute. The bulk of the Vedas deals with karma-kanda. As part of karma-kanda the Vedic texts have to be chanted or recited in a particular way, just like music. That is similar to ragas which have to be sung in a specific tone. To attain a particular goal, the Vedas prescribe sacrifices. If there is even a slight discrepancy in the chanting of these mantras one could even get an adverse result.