There are two clinical forms of smallpox. Variola major is the severe and most common form of smallpox, with a more extensive rash and higher fever. There are four types of variola major smallpox based on the Rao classification:1 ordinary, modified, flat, and hemorrhagic. Variola major has an overall historical fatality rate of about 30%; however, flat and hemorrhagic smallpox are usually fatal.2 In addition, a form called variola sine eruptione (smallpox without rash) is generally seen in vaccinated persons. This form is marked by a fever that occurs after the usual incubation period and can be confirmed only by antibody studies or, rarely, by virus isolation.3

Variola minor is a less common presentation of smallpox, and a much less severe disease, with historical death rates of 1% or less.4 Subclinical asymptomatic infections with variola virus have also been noted, but are not believed to be common.3
References:
1.Rao, A. R. (1972). Smallpox. Bombay: Kothari Book Depot. OCLC 723806
2.Hogan CJ, Harchelroad F. "CBRNE - Smallpox". eMedicine. http://www.emedicine.com/emerg/topic885.htm. Retrieved 2006-09-23.
3.Atkinson W, Hamborsky J, McIntyre L, Wolfe S (eds.) (2005). "Smallpox" (PDF). Epidemiology and Prevention of Vaccine-Preventable Diseases (The Pink Book) (9th ed.). Washington DC: Public Health Foundation. pp. 281–306. http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/smallpox.pdf.
  4."CDC Smallpox". Smallpox Overview. http://www.bt.cdc.gov/agent/smallpox/overview/disease-facts.asp. Retrieved 2009-11-14.