Tsunade is the third and last part of the three Legendary Leaf Sannins, with Jiraiya and Orochimaru. Tsunade was also the first medical ninja to be put on the field. At a young age, Tsunade had a dramatic death that made her ask for a medical unit to be implemented in the Konoha military; the death of her younger brother Nawaki, the day after she gave him the necklace which belonged to the Second Hokage. After the meeting in which she laid out her idea, which was initially rejected, another jounin pushed his support for the idea. Tsunade and the jounin named Dan became very close and fell in love. In ways she and Dan were the same, they both had a younger sibling that died in battle by enemy hands. Dan was later killed by enemy ninjas. Since then, Tsunade has had a paralyzing fear of blood.
Tsunade is a very powerful ninja, she has great strength and she is a medical specialist. She can heal almost any wound and injury. She even healed Rock Lee's injuries that he got from battling Gaara during the chuunin exams. She can also perform a special technique that disrupts the target's central nervous system. She used the technique on Yakushi Kabuto, she inserted an electric charge into his nervous system, it reversed the connection of the brains control over the body. If he were to try to move his right arm, his left leg would move, if he was to move his right leg, his left arm would move instead ect. It provides her with an opening to strike them. It is a technique not to be underestimated. In addition she also has an obscene amount of strength, even at a young age, and "one good hit" will kill anyone, as Kabuto puts it. She has such great strength that she can put a huge crater into the ground just by stomping or hitting the ground. As a side note, her summon, Katsuyu, doesn't seem to talk much. She seems to be rather shy.
After Hiruzen (the Third Hokage) died and Jiraiya refused to accept the Hokage position, Tsunade was asked to take the aforementioned spot. She became the Fifth Hokage, and the first female who accomplished that.