Brahman is the word in Sanskrit language referred to God or the Supreme self. Those who do not believe in God do believe in Higher Mysterious power which human beings have not yet understood but definitely plays a role in our life. Brahman is supposed to have that power within and is synonymous with the power. When philosophers assumes "existence exists" this existence can be called Brahman.
Brahman while emerging as Universe, gets transformed into Bramha-shakti, which is also called as Mahaa-Maayaa (the power of Brahman or God). This later devides into Eight-fold Nature(Ashtadhaa Prakruti), consisting of three attributes viz., Satva, Raja and Tama; and five elements viz., Aakaash(space),Vaayu(air),Agni(fire), Aapa(water) and Prithvi(earth). In Taittiriya Upanishad it is given that after creating Universe Brahman enters the Universe. This means that Brahman menifests as the Universe, and Brahman being infinite, still remains the same. This Brahman being all pervading is within and outside the Universe.
Aatman(Soul) also emerged from Brahman and thus considered as part of Brahman. This Soul is born as a living being, possesses physical body of this eight fold nature and the soul being part of Brahman.
Thus we the living beings are soul which resides in physical body and are basically part of Brahman. The five elements constitutes the physical body and three attributes of satva-raja and tamah guides us for the actions performed by the body. These attributes directs us towards the divine, worldly things or towards the evil actions subsequently.
The ultimate the goal of our life is to merge with God or our parent substance Brahman.