The Pinan (平安?) kata are a series of five empty hand forms taught in many karate styles. The Pinan kata originated in Okinawa and were adapted by Anko Itosu from older kata such as Kusanku and Channan into forms suitable for teaching karate to young students. When Gichin Funakoshi brought karate to Japan, he renamed the kata to Heian, which is translated as “peaceful and calm”.
The Pinan kata were introduced into the school systems on Okinawa in the early 1900s, and were subsequently adopted by many teachers and schools. Thus, they are present today in the curriculum of Shitō-ryū, Wadō-ryū, Shōrin-ryū, Kobayashi-ryū, Kyokushin Shōrei-ryū, Shotokan, Matsubayashi-ryū, Shukokai, Shindo Jinen Ryu, Kosho-ryū Kempo, and several other styles.
One of the stories surrounding the history of the Pinan kata claims that Itosu learned a kata from a Chinese man living in Okinawa. This kata was called “Chiang Nan” by the Chinese man. The form became known as “Channan”, an Okinawan/Japanese approximation of the Chinese pronunciation. The original form of the Channan kata is lost. Itosu formed 5 katas from the long Channan Kata which he thought would be easier to learn. The 5 kata were Pinans Shodan, Nidan, Sandan, Yondan, and Godan.
There lots of different views on the practicality of these Kata, from them being set of Kata designed for exercise only, to the Kata themselves being adapted to fit an exercise requirement after initially being designed to contain summary of ancient Karate Kata techniques from various unrelated Kata.
A debate on the Iain Abernethy forums regarding this subject can be seen here, it contains lots of well structured debate around the progression of techniques throughout the Heian/Pinan and the applicablity of the contents in a real fight.