Article 14 of the Constitution specifies equality before law and equal protection of law. It implies, not only that similarly placed have to be treated alike, it also postulates that unequals cannot be given equal treatment. Article 14 strikes at the root of arbitrariness. But all classifications are not bad, there are two tests to be appled for any classification, one is there any rational basis for classification and is the basis of classification has a causal nexus with the object to be achieved. The concept of equality is a multihued one and its elaboration would be heavy reading.
The Article 14 is to be read with Article 15 which prohibits discrimination by the state against any citizen on grounds 'only' of caste, religion, sex, race, and place of birth. So there can be discrimination between equals on other grounds like say merit or suitability for a post.