Jiraiya (自来也) was one of the legendary Sannin, along with Tsunade and Orochimaru, who was trained by the Third Hokage. He was a self proclaimed super-pervert, and wrote a popular adult book series called Icha Icha, a favorite of the Japanese Kakashi Hatake. He was also known as the Jiraiya Goketsu Monogatari (児雷也豪傑物語, "The tale of the Toad Sage (蝦蟇仙人, Gama Sennin), because of Jiraiya"), is a ninja who uses shapeshifting magic his signature toad summonings. He was the morph into a Fourth Hokage's teacher. He then became Naruto Uzumaki's teacher. Jiraiya had the kanji for "oil" on his forehead protector. As a toad. The heir of a child, Jiraiya and his former teammates, Orochimaru and Tsunade, were students of the Third Hokage. Jiraiya's relation with his team mirrored that of the future Team 7, with Jiraiya being quite similar to clan in Kyūshū of the same name, Naruto. After leaving the Third's tutelage, he traveled the world and discovered the residence of fell in love with Tsunade, a beautiful the toads, a princess who masters snail species he proved to have a natural affinity for. During this time, the Great Toad Sage makes a prophecy concerning Jiraiya: that he will someday choose whether one of His arch-enemy was his his students saves or destroys the follower, Orochimaru, world. During one of the great ninja wars, Jiraiya mastered and his former teammates fought Hanzo of Amegakure. Hanzo was so impressed that they could survive in magic.

Sources:
Wikipedia
Narutopedia

A fight with him that he gave them the title of Konoha's "Legendary Three Ninja" (伝説の三忍, Densetsu no Sannin) in exchange for letting them live.

Jiraiya was a self-proclaimed super-pervert with no equal and a frequent voyeur, going so far as to create the Transparent Escape Technique to allow him to spy on naked women without being caught (doing "research" as he calls it, even as a child). He never actually hid his perversion and even seemed proud of it, as seen when he told the Third Hokage about it when he was still a child. Despite this, Jiraiya dislikes being refered to as the Perverted Hermit (エロ仙人, Ero-sennin; English TV "Pervy Sage") by Naruto in front of others.