Toad

Toads are the most recurring summons to be seen in the series due to some of the main characters, Naruto Uzumaki, Jiraiya, and the Fourth Hokage being able to summon them. They reside in Myōbokuzan. Toads possess a wide range of abilities, being able to spit water or oil from their mouths and occasionally utilizing weapons in battle.

They vary greatly in size; Gamabunta, the largest toad seen so far, is as big as a building, at least 100 meters tall in fact, while his two sons, Gamakichi and Gamatatsu, were small enough in Part I to ride upon a man's head. The two have gotten much bigger since their last appearances, however, Naruto now capable of standing upon their backs.

Unlike most summons introduced in Naruto, each toad is shown to have a unique personality; Gamatatsu has a fixation for snacks, and Gamakichi enjoys watching battles from a safe distance. Their father, Gamabunta, speaks like a yakuza gangster, who won't work with anyone he doesn't respect. Despite this, he is fiercely protective of his children and Naruto, and will fight for their safety regardless of his respect for the summoner (or, more likely, whether or not Jiraiya was on his good side). Gamaken, a toad of comparable size to Gamabunta that was introduced in Part II, spent the bulk of his brief appearance commenting on how weak and stupid he is.

The toad elders seem to hold high positions in toad society; the Great Toad Sage, an aging and senile toad, hands out wisdom and predictions to the younger generations. He is highly respected for his track record of always being right, and getting to meet with him is considered a great honor. A married toad couple, Shima and Fukasaku, assist the Great Sage in his work. They also seem to be particularly powerful, as Jiraiya summoned them to help him during a battle with Pain. Jiraiya spoke to both of them very respectfully, whereas most other toads he treated as his equals. Although Shima and Fukasaku constantly bicker over such things as dinner and Jiraiya's immaturity, their combined efforts were enough to defeat three of Pain's bodies.

Summoners
The toad summoning contract has been signed by at least five people. Only three of these five have been seen in the series:

The latest summoner of toads, Naruto didn't usually summon them unless he needed Gamabunta (which ticked him off). However, upon Jiraiya's death, Naruto became more proactive with them, even going to Myōbokuzan to learn senjutsu. Naruto is highly respected among them, as their leader foretold him as a "Prophesized One".
 * Naruto Uzumaki

Nothing is known about Minato's relationship with the toads, but apparently he was well-respected among them. His only known summons were Gamabunta, whom he called to help him fight the Nine-Tailed Demon Fox, and Gerotora, who holds the key to the seal of the Nine-Tails on Naruto's stomach.
 * Minato Namikaze (Deceased)

The most frequent summoner of the toads, Jiraiya's techniques mainly consisted of toad-based abilities. Jiraiya's nickname was even "The Toad Sage". The toads had a cordial relationship with Jiraiya, often making fun of him despite their great mutual respect.
 * Jiraiya (Deceased)

Trivia

 * Masashi Kishimoto has stated that there are a number of different types of toads; the toad that appeared on the cover of volume 13 of the manga he stated to be a Shaolin kung fu master. He hopes to be able to draw many of these toads for the duration of Naruto.
 * The kanji character for is identified as the symbol of the Ninja-Toads of Myōbokuzan; a possible allusion for the Toad's signature technique, the Toad Oil Bullet technique.  This character was first seen in the series on the customized Forehead Protector of Jiraiya, who was famously known as the, after having made a pact with the Ninja-Toads and signed a summoning contract with them.  This very same kanji character was also later seen on a necklace worn around the neck of the Great Toad Sage, a highly respected prophet and the oldest known living Toad in Mount Myōboku.
 * Sakura Haruno is shown to be terrified of the toads; however, she can still work with them.