In all the four Vedas there are the three sections of karma, upasana, and jnana. The jnana section is comprised of many Upanishads. Even though there are more that 108 Upanishads in the Vedas, the knowledge taught in them is the same – the knowledge of the self.
For all of us living here in this world, questions such as, “Who am I, what is this world, and where did it come from?” eventually arise. The first question is related to myself and my own true nature, while the second question relates to the nature of the world that I see around me. Our desire for the knowledge that will answer these questions is fulfilled in the Upanishads. They tell us that Brahman is the origin of this creation and is also our own true nature.
The next question we would want to ask is what is Brahman? Brahman is that from which we all beings are born, that by which they are sustained and that unto which they return.
Source: Hindu Culture – An introduction – Page 42 - 43