Yuga Dharma is one aspect of Dharma, as understood by Hindus. Yuga dharma is that aspect of dharma that is valid for a Yuga, an epoch or age as established by Hindu tradition. The other aspect of dharma is Sanatan Dharma, dharma which is not subject to change.
Hindu sacred writings are broken into two groups: Śruti writings (such as the Vedas) regarded as timeless in character, and Smriti, writings that focus on less timeless elements. Sanatan Dharma is based on the Shruti writings, while Yuga Dharma is based on the Smitris.
Some scholars describe Santan dharma as the overall, unchanging and abiding principals of dharma, and describe Yuga dharma as a lesser aspect of dharma, since it is constantly changing. Such scholars distinguish Sanatan dharma as the dharma of religion, and Yuga dharma as the dharma of social interaction: law, ethics, etiquette and so on.
Swami Vivekananda describes the distinction between them in this way. Of Sanatan dharma, he says:
We know that in our books, a clear distinction is made between two sets of truths. The one set is that which abides for ever, being built upon the nature of man, the nature of the soul, the soul's relation to God, the nature of God, perfection and so on; there are also the principles of cosmology, of the infinitude of creation, or more correctly speaking, projection, the wonderful law of cyclical procession, and so on; these are eternal principles founded upon the universal laws of nature.
Of Yuga dharma, he says:
The other set comprises the minor laws, which guide the working of our everyday life. They belong more properly to the Puranas, to the Smrtis, and not to the Sruti. These have nothing to do with the other principles. Even in India, these minor laws have been changing all the time. Customs of one age, of one yuga, have not been the customs of another, and as yuga come after yuga, they will still have to change.