Sunscreens. The new ratings do not include a term, sunblock, any more. Sunscreens can protect against both ultraviolet a and ultraviolet b cellular damage to your skin. Some of the older ultraviolet a agents were found not to be stable in the sun! Newer generation sunscreens have photostable uva blockers. The old term of sunblock referred to the white mineral zinc oxide cream that people put on their noses.
Sun Protection. Sunscreens work via 2 possible ways: 1) chemical blocks that breakdown the uv rays as they pass through the skin and 2) physical blocks that prevent the uv rays from getting to the skin. I recommend the physical blocks (zinc and titanium) because they have a very good safety profile and work very effectively. If you are going to be in the sun - use a minimum of SPF 15 - follow product directions.
Sunscreen, use it. Sunscreen with ultraviolet a and b protection is necessary. Most importantly, most patients don't use enough of sunscreen during the application. Reapply after you have entered water.
Physical vs chemical. There's no such thing as a complete sun blocker. Sunscreens help to filter out damaging uva and uvb rays but nothing blocks them completely. There are physical blockers (ie: zinc oxide, titanium dioxide) and chemical sunscreens (ie: avobenzone). Physical blockers are more effective while chemical sunscreens look better. Look for a sunscreen with micronized zinc oxide with SPF 30 or higher.