When she was younger, Sakura's only defining characteristics as a ninja were her intelligence and excellent chakra control. After she begins training under Tsunade, Hokage of Konoha, she realises her talent for performing medical jutsus and also for genjutsu. She is taught how to utilise her chakra to the best of her ability--so by the time she is fifteen, she is hailed as being fully capable of surpassing Tsunade in the future, as a medic-nin and a kunoichi.
Sakura was once naive and judgmental, not really understanding either of her teammates--Naruto and Sasuke--very well. At first she shunned Naruto and only ever paid attention to Sasuke. As time progressed, and she began to recognise Naruto's talents and skills, she began to see past what she had originally thought of him--and is able to value him as a true friend, and somebody who will stand by her. She also begins to understand Sasuke better, and her feelings for him mature as she begins to outwardly treat him like a friend and a teammate rather than just a crush. Her feelings for him also deepen into something that is more sincere, and selfless. She considers the both of them to be two of the most important and influential people in her life.
Sakura, at first, is far behind her teammates in terms of her skills and abilities. She does not know any techniques outside of the standard Genin jutsus, and is often targeted in battle, needing protection from Naruto, Sasuke or their sensei, Kakashi. As time goes on, she begins to add more to the fights, and realises her incompetence, promising herself that she will change and become stronger. She seeks tutelage from Tsunade because of this, and by the time she's fifteen, she is a formidable and skilled ninja, who has passed the Chunin exams without the aid of her original team.
She also matures in these formative years. She is no longer naive and unaccustomed to the reality of being a ninja, and is a lot more emotionally sensitive to those around her. She has left her weak self behind, and is now a loving, caring person who fights for what she believes in and cares about. She is motivated by Naruto, whose unfaltering dedication and optimism have inspired her, and also Sasuke, who she promises to bring back to Konoha. Her inner self, dubbed Inner Sakura, has also seemingly disappeared--she is now not afraid to voice her opinions, or what she thinks, and as a result can also be rather violent. But she is now comfortable in her own skin, and works for her goals rather than staying in the background like she did as a Genin. She is no longer inexperienced and as immature as she was.