Brahman is the Sanskrit word for the Ultimate Reality, the Supreme Being, or the Absolute. It is unchanging, unborn, unmanifest, and infinite. But Brahman is both transcendent and immanent; the ground from which all of manifest reality arises, including time and space. Brahman is omnipresent and is the spiritual source of the universe.
In Hinduism there are different interpretations as to the ultimate nature of Brahman depending on whether the texts are non-dual or dualistic, or variations of the two.
Brahman is also sometimes personified as the creator god Brahma, and is seen alongside the two other divine functions in the Trimurti – Vishnu the preserver, and Shiva the destroyer.
Atman is a Sanskrit word meaning self or breath. It is your basic essence as an individual before you become identified with changing phenomenon. Atman is the true Self as opposed to the personal self identified with the body and mind.
In Hinduism, the Atman is the universal self which reincarnates until it achieves moksha, or liberation from samsara. As Brahman is to the workings of the universe, so the Atman is to the workings of your being as a person.
Atman is your little spark of Brahman. In some systems of thought, Atman and Brahman are considered to be one.