Sanatan Dharma is the real name for what we call today as Hinduism. Sanatan means eternal, which never begin and will never be ending, one absolute without the other. Sanatan Dharma is the eternal and absolute set of duties which binds all Hindus regardless of any caste, color, creed or sect. Dharma came of from the sanskrit word (Dhri - धृ) which means to hold together. Hence Sanatan Dharma is the main culture and the Hinduism is the way of life. Sanatan Dharm is a Perennial Philosophy, Universal Tradition, All-Pervading Truth, Natural Flow and ever existed.
Sanatan Dharm is the main religion found in the Indian Subcontinent. It prevails instead of the fact that a lot of conversions had happened for socio-political reasons. Still a lot of people believes in Sanatan Dharm. As people of Iran pronounces "sa" as "ha", they called the people lived across Sindhu Valley Civilization as Hindu and hence the religion later on is referred as Hinduism. We can only find the word Hindu in the scriptures for around 1000 years but the Sanatan Dharm was based on Vedas. With reference to Sanatan Dharm Hinduism is the base of all the religions in the world. This is the main reason why Sanatan Dharm is referred as a greatest scientific belief.
Hinduism has more then one doctrines that defines its core principles. There are some shared thoughts across all doctrines. For eg there are Shaiv sects of Hinduism and Vaishnav sects of Hinduism and both form tilak in three lines on there forehead - The only difference is that Shaivs do it vertically and Vaishnavs horizontally.