It is a grand collection of puranas and spiritual stories with lots of philosophical teachings too. It contains the stories of Bhagvan Vishnu’s 24 avatars and stories of several great devotees. Srimad Bhagavatam is considered the greatest of all the puranas because it contains all the three shades of Hindu philosophy (Advaita, Vishitadvaita and Dvaita) in its stories, discourses, dialogs, prayers and parables. The life story of Lord Krishna occupies a prominent position in this purana. It is one Purana that can be said to bridge both Bhakti (devotion) and Jnana (Knowledge) in a wonderful way. Vyasa Maharshi was the author.
Bhagavad Gita is essentially philosophical teachings of Lord Krishna, delivered as sermon to Arjuna just before Kurukshetra war and it is part of Mahabharata . Mahabharata too was a book authored by Vyasa Maharshi. But the Bhagavad Gita section is out and out a book of spiritual teachings and not a story book. It predominantly teaches Karma Yoga and also covers Bhakti and Jnana yogas. Bhagavad Gita contains the essence of the teachings of Upanishads in a simplified way.