The Hindu Gods are portrayed with multiple arms to indicate that God is multi-tasker or probably a multiprocessor which has to act as per events happening in multiple parallel universe.
The multiplicity of arms emphasizes the deity's immense power and ability to perform several acts at the same time.
The multiple arms are to show a difference but also a greater power than humans. The number of arms varies depending on the symbolism being shown. Thus normally it is 4 arms each holding a different item (with a different meaning).
The portrayal of a deity in human form but with multiple arms is the artist's attempt to express the deity's superhuman power. Demons are frequently depicted with multiple heads to convey their superhuman power as well.
Multiple heads may indicate that God is observing each parallel universe independently.
Each arm of the Gods hold a different divine object. For example, Lord Vishnu carries a conch shell in one hand, a mace (Gada) in another. His other two hands carry a lotus and the Sudarshana Chakra.
Ganesha often carries similar items to Vishnu. Ganesha has an elephant head as his head was accidentally cut off by his father, Shiva. In order to alleviate Shiva's wife, Parvati of her grief, he set out to find a new head for the boy.
Ananta is a serpent with many heads to symbolize infinity.
Kali is usually depicted with three eyes to symbolize sun, moon, and fire as well as her ability to see into the past, present and future.
The cosmic manifestations of God are meant to accentuate various characteristics God owns.
Often these characteristics are visualized with images of God having multiple heads.
Bramha with four heads representing 4 divine Vedas.
Pseudo secular anti-hindu fanatics in social networking sites and media who do not hesitate to ridicule Hinduism of worshiping snakes, monkeys, cows etc. have no idea of what our Upnishads and Puranas teach.
Let their ignorance overwhelm their senses.