Shafi'i is one of the four schools of Fiqh or religious law within Sunni Islam. Shafi'i is practiced throughout the Ummah, but is most prevalent in Egypt, Indonesia, Thailand, Singapore and the Philippines, and is the school of thought officially followed by the government of Brunei Darussalam and Malaysia. It is followed by approximately 15% of Muslims world-wide. 
The Shafi'i tradition is particularly accessible to English speaking Muslims due to the availability of high quality translations of the Reliance of the Traveller.
Muhammad Ibn Idris (150H - 204H) (767-820) (Imam Shafi'i) is an Arab born in Gaza, Palestine. Once a scholar to Imam Malik in Medina and Imam Muhammad Ibn Hassan in Baghdad.
The Shafi'i school of fiqh is based on Al-Quran (Koran), As-Sunnah, Al-Ijma', his companion's opinion (mostly Al-Khulafa Ar-Rasyidin) and Al-Qias. His famous books are Ar-Risalah and Al-Umm, they emphathize on instibat(firm law).
Shafi'ism is considered to be one of the more conservative of the four schools of Islamic jurisprudence, but there are many adherents of the Shafi'i tradition who maintain liberal views in practicing their religion.
The Shafi'i school of fiqh is named after its "founder": Muhammad ibn Idris ibn al-`Abbas, al-Imam al-Shafi`i, Abu `Abd Allah al-Shafi`i al-Hijazi al-Qurashi al-Hashimi al-Muttalibi (better known as Imam Shafi).