Some folks rate ND filters by transmission density: .1, .2, .3, etc. Each point is 1/3 of a stop. So a .3ND filter is one stop. A .6 is two stops.
Others use filter factors: ND2 (one stop), ND4 (2 stops), ND8 (3 stops)
There is a chart to help at
http://www.photofilter.com/neutral_density.htm
more at
http://dpfwiw.com/filters.htm#nd and
http://dpfwiw.com/filters.htm#factors
John