Short answers, pulled from the story.
In Buddhism, āgama is a Sanskrit and Pali term meaning "sacred work" or "scripture," referring to a collection of early Buddhist discourses attributed to the Buddha and his early disciples. The term is functionally equivalent to nikāya as used in the Theravada Pali Canon, though the word āgama does not itself appear in that collection.
The āgamas and the nikāyas are parallel but not identical collections. Large portions of the Anguttara Nikāya and Samyutta Nikāya have no counterpart in the āgamas, and several individual sutras differ in content between the two traditions. Scholars describe the āgamas as a "Northern Collection" and the nikāyas as a "Southern Collection."
The four extant āgama collections survive in their entirety only in Chinese translation. Small portions of all four have also been found in Sanskrit, and parts of four of the five āgamas survive in Tibetan translation. Fragments also exist in Gandhari and other ancient Indic languages.
At minimum the Sarvāstivāda, Kāśyapīya, Mahāsāṃghika, and Dharmaguptaka schools each held their own recensions of the āgamas. Each school's version could differ from the others in language, content, and the number of texts included.
Saṃghadeva translated the complete Madhyama Āgama of the Sarvāstivāda school into Chinese in 397-398 CE during the Eastern Jin dynasty. That version contains 222 sūtras, compared with the 152 suttas in the Pali Majjhima Nikāya.
The Ekottara Āgama is the most divergent of the four surviving Sanskritic āgamas from its Theravadin parallel. According to scholar Keown, more than two-thirds of the sūtras are found in one compilation but not the other, suggesting much of the collection was formed at a relatively late date. It also contains variants on core teachings such as the Noble Eightfold Path.