Seals (印, in?) 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.