Brahman is the Sanskrit word for ultimate reality or eternal origin. Brahman shouldn't be translated as 'God', as Brahman is not worshipped - rather manifestations or forms of Brahman are treated as gods.
There are three main gods, or forms of Brahman, as worshipped by Hindus.
Brahma (the creator)
Don't confuse Brahma with Brahman! Brahma is the god of creation - of this world and all its creatures. Often depicted as a four-armed, four-faced god, Brahma is the least worshipped of the triumvirate - as his work is considered complete.
Shiva (the destroyer)
Shiva is responsible for the cycle of the universe - the continual creation, preservation and dissolution of the universe. Don't think of Shiva as an evil, violent or malevolent god. Rather, Shiva is the god of mercy and compassion, guiding worshippers away from greed and lust.
Vishnu (the preserver)
Vishnu is responsible for the preservation of the world - both physical and spiritual. Vishnu is incarnated when the world is troubled (nine times so far, with the final incarnation to occur just before the end of this world). He is tasked with returning the world to peace and order.
In Hinduism, the heavenly triumvirate consists of Brahma, Vishnu, and Shiva. Collectively, they are the Trimurti. Each of the three gods is also known by their individual disposition or guna.
For example, Brahma is known as guna-rajas (the passion that brought the world into being). Vishnu is known as guna-sattva (the embodiment of mercy that preserves the world), while Shiva is known as guna-tama (the embodiment of wrath, the destroyer of all evil). In Hindu temples, the triumvirate is shown as three gods in one; Brahma may be situated on the right, Shiva in the middle, and Vishnu on the left. Here, it is important to note that, in temples, the physical position of each god in the trimurti is interchangeable. What does not change, however, is that the three gods will always be known for their respective roles of creator, preserver, and destroyer.
Originally, the three gods of the triumvirate were equal; none took precedence over another. However, eventually, Shiva and Vishnu came to be more revered than Brahma himself. It is said that Brahma created all things himself, from the sun and the moon to the various castes in India. However, Hindus today prefer to concentrate on Vishnu and Shiva, as these two gods continue to work on man's behalf (as opposed to Brahma, whose work of creation is finished).
Source: The Gods of India: A Brief Description of Their History, Character & Worship by E. Osborn Martin