Jutsu, or skills/techniques, are the mystical arts a ninja will utilize in battle. To perform a jutsu, the ninja will manipulate their chakra and may use some form of hand seal. Some techniques like Taijutsu do not require hand seals, however the ninja may use them to focus their chakra to certain areas of their body. Also some Ninjutsu like Rasengan and various weapon utilizations do not require any hand seal use.
By forming hand seals, the ninja is able to mold their chakra and manifest the desired jutsu. Because of the extensive number of hand seals and different combinations, there are thousands of potential jutsu to be discovered and used. The hand seals themselves come from the twelve signs of the Chinese Zodiac. The appearance of the seals stem from Hindu and Buddhist iconography, and are called "Mudra" meaning "seal" or "sign" in Sanskrit. These hand gestures helped identify the nature and function of the deity who used them. Over time the positions and traditions changed as they were adopted and moved from country to country.
In addition to these hand seals, certain jutsu have their own unique hand and finger positions, such as the gestures used in Kage Bunshin, Shintenshin, etc.. Every ninja in the series save for one has used both hands to form seals. Haku used one-handed seals in his Hyouton bloodline limit techniques. It is unknown if this was a skill unique to the whole of the bloodline or merely Haku himself. Note this is different than the one handed chakra focusing gesture one sees a lot of ninja use.