Sakura Haruno is a medical ninja from Konoha. After Sasuke left Konoha to joins Orochimaru, and Naruto also left Konoha to train with Jiraya. Sakura decided to learn from the 5th Hokage, Tsunade. But Tsunade often busy with her work as a Hokage. So, Tsunade appointed you train Sakura. (The timeline happens in between Naruto and Shippuden)