Shiva and Vishnu are not regarded as ordinary devas but as Mahādevas ("great gods" ) due to their central positions in worship and scriptures. These two along with Brahma are considered the Trimurti—the three aspects of the universal supreme God. These three aspects symbolize the entire circle of samsara in Hinduism: Brahma as creator, Vishnu as preserver or protector, and Shiva as destroyer or judge.
There is only one ultimate Deity, it is normally considered Brahma The Creator. Also known as Atman, all others are considered his avatars- Vishnu The Protector or Preserver, and Shiva The Destroyer are the main examples of this, due to them being the most popular Avatars of Brahma. In their personal religious practices, Hindus may worship primarily one or another of these avatars, known as their "Ishta Devata", or chosen avatar. The particular form of God worshiped as one's chosen ideal is a matter of individual preference, although regional and family traditions can play a large part in influencing this choice. Hindus may also take guidance about this choice from scriptures.
Although Hindus do worship avatars other than their chosen avatar from time to time, depending on the occasion and their personal inclinations, it is not expected that they will worship, or even know about, every form that Brahma has taken. Hindus generally choose one concept of Brahma (popular choices include Krishna, Rama, Shiva, or Kali), and cultivate devotion to that chosen form, while at the same time respecting the chosen ideals of other people.