Sunscreen vs sunblock

These two words have very similar definitions on Google. One protects against UVA and one protects against UVB rays. The question is, at what times should I use one word, and the other? Or can they be used interchangeably?

For many speakers, sunscreen and sunblock are completely interchangeable.

Here are the definitions according to Oxford Dictionaries:

Sunblock

A cream or lotion for protecting the skin from the sun and preventing sunburn.

Sunscreen

A cream or lotion rubbed onto the skin to protect it from the sun.

Other dictionaries also give synonymous definitions.

Any definitions which attempt to draw a technical distinction, say between protection from UVA rays and protection from UVB rays, is likely going beyond common usage. This is not a distinction most laypeople draw, since most laypeople are not in possession of the concepts UVA and UVB rays.

That said, some laypeople still draw a distinction between the two substances, with sunscreen being a weaker, less opaque versions of sunblock. These connotations can be seen in the respective root words screen and block. The idea is that sunscreen merely screens you from the sun, while sunblock completely blocks it out.

For this reason, the Food and Drug Administration (FDA) seems to have issued technical definitions, as well as regulations regarding the use of the terms on products. Here is a relevant passage from Wikipedia:

The use of the word "sunblock" in the marketing of sunscreens is controversial. Since 2013, the FDA has banned such use because it can lead consumers to overestimate the effectiveness of products so labeled. Nonetheless, many consumers use the words "sunblock" and "sunscreen" synonymously.

This passage seems to confirm that many speakers use the terms synonymously, although it also acknowledges the existence of connotations of strength.

Sunscreen seems to be about 6 times more frequent than sunblock, as evinced by this ngram.

In short, the answer to your question is the following: Many speakers use sunscreen and sunblock interchangeably, but many others use them with different connotations of strength.