What are the main symbols of Hinduism?

The main symbols of Hinduism are Aum (Om), Swastik, Shree, Trishula, Shanka, Chakra.

The most common symbols are 'Om' and 'Swastika'. The other could pertain to the chosen deity of the person, for example, the trident for Shiva, a power triangle for the mother Goddess, Durga, a conch for Vishnu, etc.
'AUM' or 'OM' (pranava) is the most pronounced sacred symbol which means "the essence of entire universe".
However, it must be realized that every Hindu idol is a symbolic representation of a basic force and thus a collection of symbols.
Thus, Vishnu, the preserver, has four important symbols viz. Chakra, Conch, Lotus and Mace.
These four symbolically represent four different properties of the semi voluntary force, which Vishnu represents.

1. Chakra symbolizes the frequency of this cyclical force
2. Conch indicates that the force makes life as flexible as water
3. Lotus indicates that an activity can be executed with this force without affecting any other, just like lotus stands out of water.
4. Mace symbolizes the amplitude of this force.

In Hinduism, the sound "Om", also called the 'Pravana' or 'Udgita', is considered as the verbal representation of God, who can not be described. In Hinduism, the Absolute God is nirguna (with out qualities), nirakara (with out form), nirvisesha (without attributes), but for the sake of human comprehensive God is worshiped in various forms. All these gods can be considered as "symbols" of the One God!