Sun protection, what are the characteristics?

You’re lost in front of the sunscreen aisle. Numbers are highlighted on the tubes, but which one to choose?
The SPF, or Sun Protection Factor, is an index indicating protection against UVB and UVA rays:

6 or 10: low

15, 20, or 25: medium

30 or 50: high protection

50+: very high

The SPF also represents the ratio between the time needed to get a sunburn with and without the product. For example, if a person gets sunburned after 10 minutes without protection, with an SPF 15, it will take 15 times 10 minutes (i.e., 2 hours and 30 minutes) to get the same sunburn.

Also read:

UVA, UVB, why are they harmful?
Sun protection, what are the characteristics?
Which sunscreen to choose?