There are many sects in Islam, but only two major ones: Sunni (90%) and Shia(10%). Shia in turn are divided into many different ones. Sunnis don't have any subdivisions. The salafist/wahabist movement within Sunnism stresses that people take everything in Islam literally and follow a very strict interpretation. When we talk about radicalism or a certain percentage of Muslims in the world wanting apostates being punishable by death, we are largely referring to sunni muslims.