The etymological meaning of dharma is 'that which binds together'. So it is that which is inherent as law in the very nature of all existing phenomena, that which supports and holds the universe together. It is not merely a set of beliefs having no connection with living, but rather a set of principles for a harmonious and beneficient life. It is a practical doctrine.
Dharma is constant, but its form varies from situation to situation. Conscious action in accord with the reality of the moment is Dharma. Learning to act consciously is learning to act in accord with the principles of the cosmos. So Dharma is that action which accords with cosmic knowledge. In the "Bhagavad Purana" [a famous holy hindu scripture] "eating, sleeping, mating and defending are known as being common to human beings and animals, but Dharma is human beings' special prerogative."
There are ten significations of Dharma and all of them are neccessary to act in accordance with the it's law. They are : firmness, forgiveness, selfcommand, restraint (i.e not stealing), cleanliness, control of the sense and work organs, intellect, right knowledge, truth and absence of anger. All these are a part of it.
Dharma is a particular code of conduct, but there are no do's and don't as such because, whatever you feel is genuinely good for yourself is good for others also. There is nothing like doing good for others. There is no Adharma or vice, worse than causing harm to another. Dharma can best be understood when related to conduct, to morality and ethics, but these are only attributes, reflections, but not Dharma itself.
The Dharma of fire is to burn. That of water is to quench, as it is it's to create taste. It is the Dharma of fire to creat colour and form. The innate and essential nature of a thing is its' Dharma.
Though Dharma exists always, beyond form, it assumes a special form for each individual. For example it is the Dharma of an individual who lives in a cold climate to wear warm clothing, of the hungry man to have food, of the student to study properly. Of the child to play freely, without care for the world of the spirit. Of the elderly to behave soberly and dwell in the realm of spirit. Similarly of a mother to take care of her child. So Dharma is the eternal truth which holds all existence in proper relationships. There is no escape from it, because the moment there is an escape from Dharma, there will definitely be a downward flow of energy and that is Adharma.
Also read the much more detailed text "Dharma", by Kathia Baba.
A wonderful story on this subject is "The Saint and the Scorpion".
Also read "The Calendar Poem" by Harish Johari on the subject of Dharma.