Sanatan, means Eternal and Universal. Dharma, is a collection of practices based on the Vedas.
Sanatan literally means "no beginning no end". we need to understood this across three dimensions - Time, Space, and Causation.
On the time dimension implies that it is ETERNAL. In other words, there was never a time when it did not exist. It existed at all times - before creation (Srishti) and after destruction (pralayam).
On the space dimension it implies that it is UNIVERSAL. Per the vedic scriptures, there are billions of such universes each consisting of 14 planes of existence that sustain intelligent life forms. The 14 planes (Loka) include the 7 Northern loka's - Bhoo:, Bhuva:, Svarga, Maha:, Jana: Tapa: and Satyam; and the 7 Southern loka's - Atala, Vitala, Sutala, Mahatala, Rasatala, Talatala, and Patala.
The Causation dimension implies that it was not created by an individual being. Even God, the Brahmam, did not create the Vedas, the body of knowledge that form the basis of Sanatan Dharma.
What is Dharm?
Dharm, is defined as "Veda Bodita Ishata Sadanam". Firstly, Dharm is a practice that has be described in the Vedas. It has to be followed as described. When done so, it is supposed to bestow the desires of the person who performs it. Activities not described in the Vedas are not Dharm, even if they are Just, Legal, and Humanitarian. For Example, donations given to humanitarian causes do not fall under what is described in the Vedas as "daan". On the other hand, the vedic practice of "marriage" in which, the father performs the "kanya daan" of his daughter to the groom, is a Dharmic practice. Similarly, there are specific procedures defined in the vedas for donation of cow (Go Daan).
The other interesting aspect is that the procedure described in the vedas do not change wherever in the universe it is practiced.
