The Brahmanas are part of the Hindu sruti (revealed knowledge) literature. Each Veda has a Brahmana attached to it, which is essentially a collection of texts with commentaries on the particular Veda. They are usually a mixture of legends, facts, philosophy and detailed explanations of Vedic rituals.
They also consist of instructions as to how to properly conduct rituals and enunciate the science of sacrifice. They also explain the symbolic significance of the sacred words used in the rituals.
Although historians disagree on the dating of the Brahmanas, it is usually pegged to be composed and compiled between 900-700 BC.
As mentioned above, each Veda has its accompanying Brahmana.