When God is thought of as the supreme all-powerful person (rather than as the infinite principle called Brahma), God is called Īśvara or Bhagavān. Īśvara is a word used to refer to the personal aspect of God in general; it is not specific to a particular deity.
Īśvara transcends gender, yet can be looked upon as both father and mother, and even as friend, child, or sweetheart. Most Hindus, in their daily devotional practices, worship some form of this personal aspect of God, although they believe in the more abstract concept of Brahma as well.
Sometimes this means worshiping God through an image or a picture. Sometimes it just means thinking of God as a personal being.
Depending on which aspect of Īśvara one is talking about, a different name will be used—and frequently a different image or picture. For instance, when God is spoken of as the creator, God is called Brahmā. When spoken of as preserver of the world, God is called Vishnu. When spoken of as destroyer of the world, God is called Shiva.
Many of these individual aspects of God also have other names and images. For example, Krishna and Rama are considered forms of Vishnu. All the various deities and images one finds in Hinduism are considered manifestations of the same God, called Īśvara in the personal aspect and Brahma when referred to as an abstract concept.
In their personal religious practices, Hindus worship primarily one or another of these deities, known as their “ishta devatā,” or chosen ideal. The particular form of God worshipped as one’s chosen ideal is a matter of individual preference. Regional and family traditions can influence this choice. Hindus may also take guidance about this choice from their scriptures.
Although Hindus may worship deities other than their chosen ideal from time to time as well, depending on the occasion and their personal inclinations, they are not required to worship—or even know about—every form of God. Hindus generally choose one concept of God (e.g., Krishna, Rama, Shiva, or Kali) and cultivate devotion to that chosen form, while at the same time respecting the chosen ideals of other people.