Raghuvamsa is believed to be a lineage of warrior kings tracing its ancestry to Surya. Kalidasa's famous work, Raghuvamsha describes the greatness of this race. This was a race which produced great kings like Harishchandra, Dileepa, Raghu, Aja, Dasaratha and Rama. Lord Rama's father was Dasaratha, who was born to Aja. Aja's father was the great king Raghu, after whom the dynasty was known as Raghuvamsa, or the dynasty of Raghu. Raghu's father was emperor Dileepa.
Aja, was a valiant and loving son to Raghu, as Raghu was to Dilipa. Aja did many things in his lifetime like freeing a Gandharva (A Celestial Being), from a curse.
Aja was married to Indumati who was the sister of King Bhoja. Aja was given a spear when he freed the Gadharva, the spear had the power to put enemies to sleep. He used this spear in a battle, when he was defending Indumati. Aja was a great king, like his grandfather Dilipa, his father Raghu, his son Dasharatha, and his grandsons, Rama and Bharata. (Aja had 4 grandsons but two were made kings (Bharata was king when Rama was exiled to the forest for 14 years).
At the time of marrying Kaikeyi, he assured her and her family that any son born to her would supersede his other sons and succeed him as king of Ayodhya. However, none of his three wives bore children for many years. Dasharatha then performed a sacrifice, the putra-kameshthi yagna, to secure the birth of a son. Four sons were the born to him: to Kaushalya was born Rama, to Kaikeyi Bharata, and Sumitra was blessed with twin sons, Lakshmana and Shatrughna. The story of the lives of his children is the story of the Hindu epic, the Ramayana.
Dasharatha was a young man when he accidentally killed Shravana Kumara while hunting in the forest one day. Shravana Kumara's parents cursed Dasharatha, saying that he too, like them, would die of longing for a much beloved son. This is what came to pass; Dasharatha died soon after Rama left for exile in the forest, accompanied by his wife Sita and his brother Lakshmana.