METAR: CYYC 190800Z 34018KT 1 1/4SM -SN DRSN BKN010 OVC021 M09/M10 A3005 RMK SN4SC3SC1 /S01/ RVR RWY28 4500 FT SLP238
Snow is obscuring 4/8ths of the sky, the lowest stratocumulus layer is obscuring another 3/8ths, and the higher stratocumulus layer is obscuring 1/8th, making a total of 8/8ths of the sky being covered.
"931" groups indicate the total snowfall/frozen precipitation amount fallen in the past 6 hours. For example, 931023 would mean 2.3" of snow/sleet/etc. had fallen in the past 6 hours preceding the observation.
Here is the book observers use in U.S.:
http://www.faa.gov/airports_airtraff.../media/SWO.pdf
There have been some amendments (like the "931" group), which haven't been published, but the book should give you a good idea. Note the remarks in METARs are not in the order they are presented in the book, that is because the ASOS software does not, unfortunately, coincide with what is published in the book.