What a wood shingle is
A sawn, tapered piece of wood, smooth on both faces, made to precise sizes and sold in grades. Shakes are the rougher, split cousin. Most shingles are western red cedar because it resists rot naturally and is available in long, straight, clear lengths. White cedar, redwood and cypress are also used; pine and other rot-prone species need treatment and careful installation.
The same shingles are used as wall siding, which is where you see them most in modern construction.
How they weather
New cedar is warm brown; within a couple of years in our sun it silvers to gray. That is normal. Problems start when the shingles cup, split or go soft from moisture held by debris, or when moss and lichen take hold on shaded slopes. In the dry inland air the main wear is from sun and wind rather than damp.
Untreated cedar can last a long time in a mild climate; in valley heat it dries and splits sooner.
Maintenance
Keep the surface clear of leaves and needles so it can dry. Trim overhanging trees. Have loose or split shingles replaced individually rather than letting a slope go. Some owners apply preservative or fire-retardant treatments periodically, which needs to be done properly to be worth anything.
It is the highest-maintenance roof in this post series.
The fire problem
Much of San Bernardino County, particularly the foothill and canyon neighborhoods, is in a designated fire hazard zone requiring a Class A roof assembly. Untreated wood shingle is not Class A. Pressure-treated fire-retardant shingles exist and can meet the rating as part of a tested assembly, but they cost more and the treatment fades.
Many insurers in these areas will not write a policy on a wood roof at all.
What people replace them with
Luxury asphalt shingle in a shake profile, concrete tile in a flat shake-style profile, or a shake-look metal panel. All three give a version of the appearance with a Class A rating and far less upkeep. If you have a wood roof and are being told by your insurer to change it, those are the three conversations to have.
I can show you samples of each against the house.