There are many deities in Hinduism, but the three central gods are Brahma, the god who created the earth, Shiva, the destroyer god, and Vishnu, the god of life. Different people regard different gods as being the most important, and some Hindus assign different roles to Brahma, Shiva, and Vishnu. Additionally, some of the gods have roles that may seem contradictory to non-Hindus. Shiva, for example, is the god of destruction, but it is precisely this destruction that creates the possibility for new life to emerge. Vishnu has a broad array of avatars, or incarnations, that figure prominently in Hindu theology. So not only are there many gods outside the three major deities, but the three gods have a multiplicity of meanings.
As mentioned in the answer above, the trio of Brahma, Vishnu and Shiva are three very important of the Gods in the Hinduism. Actually, they may be considered as some kind of super gods, representing the process of creation, sustenance and destruction of the universe. As per Hindu belief, the universe goes through a repetitive cycle of creation of material universe from nothingness, its evolution through various stages of existence, and finally its dissolution back into nothingness. The three super gods represent the forces that are responsible for these three stages of creation, sustenance and dissolution (or destruction). It should be note that these three super-gods themselves are considered to be three aspects of an ultimate power called "Brahm". Please not that though names Brahma and Brahma sound very similar, they represent very different gods or power. Brahma, which is the ultimate reality beyond the physical universe is the central pillar of the basic Hindu philosophy. It cannot be explained in short. Just to give an idea of its complexity, I will state only two of its attributes. The Brahm is infinite as well as nothingness or zero at the same time. Sounds impossible and highly unscientific. If we think of the widely accepted scientific theory of "big bang' explaining the creation of this infinite universe from nothing, it may be easier to accept the possibility of these beliefs of Hinduism being supported by some credible logic.
In addition to these three super-gods of Hinduism, who are beyond the material universe, and continue to exist between the period of dissolution of the universe and its recreation, there are another class of gods that are supposed to be part of the material universe.
These gods having existence within the universe primarily represent the natural forces and resources such as rain, wind, sun, mountains, rivers, fire and land. There is no set rules or protocol for seniority between these Gods. Though "Indra", the god of rains, is designated as "king of Gods" he does not enjoy much importance in terms of being powerful or of being worshiped by people.
It is interesting to note that Hinduism appear to consider all types of resources, major or minor as gods. Thus they also worship things like trees animals, tools and food. Ancient Hindu tradition refers to some 330 million gods. Many Hindus believe that these 330 million Gods represent the estimated human population of that time. Thus each individual is also considered a resource and therefore is a god.
There is another class of Gods, which are either carnation of the Brahm, at times represented by the super-god Vishnu, or created by the super Gods for specific task in the interest orderly conduct of the affairs of the universe. Actually it is this class of Gods that are most popular in terms of ritualistic worship by Hindus. The relative popularity of individual god among Hindus varies with various factors like region, and sect. I am giving below a random list of Gods that, according to my personal observation worshiped most among Hindus.
- Krishna - An incarnation of Vishnu. Radha, a prominent childhood friend of Krishna is also worshiped along with Krishna.
- Rama - An incarnation of Vishnu. Seeta, wife of Rama, and also incarnation of wife of Vishnu, is worshipes along with Rama.
- Hanuman - a helper of Rama in the form of a monkey. Some believe Hanuman to be an incarnation of Shiva. Here it is interesting to note that as per Hinduism beliefs, Shiva is a great worshiper of Rama and Rama is a great worshiper of Shiva.
- Shakti or enrrgy in different female forms. All these forms are generally referred as "mother". Some of these goddesses are Parvati, Durga, and Kali.
- Ganesha - son of Shiva with the head of an elephant.
It is interesting that worship of Brahma, one of the three super gods, and Indra the king of gods is almost non-existent among Hindus.