Seals (印, In, English TV: Hand Signs) are used to perform many ninjutsu, genjutsu, and other secret arts other than taijutsu. The exact amount of chakra necessary to perform a technique is manipulated through hand seals. There are different sequences of hand seals for every technique, requiring memorisation.
While techniques may require a number of hand seals to work, a skilled ninja can use less or even one to perform the same technique. An example of this is the Water Release: Water Dragon Bullet Technique, which requires a total of 44 hand seals to activate. Tobirama Senju, the Second Hokage, only needed to use one hand seal to complete the technique, a testament to his skill. In fact, by Part II, Sasuke Uchiha has managed to master the Chidori to the point of not needing a single hand seal to invoke it and could use Fire Release techniques with only one hand seal. By reading the hand seals of the opponent, a ninja can predict the nature of the technique to be performed, which is virtually unavoidable if read by the Sharingan.
Although rare, it is possible for a shinobi to be able to do hand seals with just one hand. The only shinobi seen so far capable of this are Zabuza Momochi, Haku, Itachi Uchiha,Guren, Minato Namikaze, Naruto Uzumaki, and Sasuke Uchiha. It is also possible for shinobi to form hand seals with one of their hands while using the hand of another person as a substitute for the opposite one. This can happen voluntarily, like when Naruto Uzumaki and Sasuke Uchiha joined hands to dispel the Infinite Tsukuyomi, or forcefully, such as Anko Mitarashi using Orochimaru's hand for the Twin Snakes Mutual Death Technique, or (in the anime) Obito Uchiha using Kakashi Hatake's for the Fire Release: Great Fireball Technique.
Even rarer than one-handed hand seals, are shinobi who have abandoned this traditional practice altogether, usually because their fighting style revolve around speed and quick reflexes and have developed techniques that require no hand seals to complement this fighting style.
Certain elemental releases often end in the hand seals they are affiliated with. For example, Fire Release often ends with Tiger, Earth and Wood Releases often end with the Snake hand seals.