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Buddha is often seen sitting on a lotus, wearing yellow cloth. His hands are in the protective and boon giving positions. He has short curly hair with a knot. His earlobes are long.
Vishnu decided to free the world of the arrogance and oppressive rule of the Brahmin priests. The Hinduism had to be pure again of all false rituals.
Vishnu appeared as Buddha, the ninth avatar.

An other view is the next one:
The Hinduism lost a lot of followers to Buddhism. Especially people of the low castes choose for the teachings of Buddha. The Brahmin priests were losing their influence and power.
So the Brahmin priests came with the next explanation: Vishnu had come to earth in the shape of Buddha to teach a false belief. This was the way to see who the true believers were.

Siddhartha Gautama was born in 566 B.C. He was the son of a king of the Shakya-clan in present Nepal. He got married with princess Yasodra when he was 16 and he had one son. On the few occasions that he came out of the royal residence, he saw an ill man, a dead man and an old man. He was disturbed by seeing it. This could also happen to him. He was troubled by how to escape from this.
He left home when he was twenty-nine in search of the answer to life.
First he tried to find this following the way by ascesis. Afterwards he tried the way of knowledge. But he failed again. Then he tried the way “in between” (Buddhism)
After about six years he went to Bodh Gaya and sat under the Bodhi tree. While meditating he was tempted by the demon Mara, who offered him all the desires of the world. Not taking any of these temptations, he received enlightenment.
From that moment he was called Buddha (the enlightened one).
Through meditation he could see the sings as they really were.

He had to follow 4 stadia of increasing understanding (jhanas) to achieve Enlightenment.

The heart of the Enlightenment. (Teaching of Buddha) are the four Noble Truths

• The truth of suffering (dukkha)
• The truth of the origin of suffering
• The truth of stopping the suffering
• The truth of the path that leads to the end of suffering

After finding Enlightenment and preaching over 45 year he retreated in the forest and went to Nirvana. He never saw himself as a god or mediator between god and men. He was more a doctor, who recognised diseases and prescribed drugs.
Buddha never denied the presence of God. He even said that it would be very wise to call for the help of gods and goddesses. Help of the gods and goddesses would be more than welcome. It was god himself in the form of Brahma who encouraged him to share his knowledge with other people.
Most of the Hindu gods are integrated in Buddhism even the old Hindu gods like
Yama, Indra etc. Indra in Buddhism became Shakra, god of heaven.
Sharaswati became goddess of education. Shiva and Parvati lost their important
Place and became gate guardians of Buddha. Yama became Dharmaraja, god of the underworld.
Kubera became Jambhala, guard of Buddha.

A new group of myth arose: The Jataka.
It is believed that the soul of Buddha had many lives before Siddhartha Gautama found Enlightenment. A lot of folk tales were adjusted to Buddha. The stories tell us the former lives of Buddha as a rabbit, antelope, vulture, merchant, robber and god.
In each life he reached the highest level of that life. By reaching this level he was reworded to an incarnation (life) of higher scale. And so he achieved enlightenment.

547 stories in total are originally told by Buddha. These stories are written in Pali.
He lived in the fifth century B.C. and he criticised the Brahmin priests. Mainly to all the rituals and offerings. These rituals and offerings lead to exploitation of the poor. The Brahmin priests used the god to extend the illness and by doing so they earned more money.

Not only Buddha criticised the Brahmin priests. The Jainism did also criticise them.