I'll discuss "radiation and radioactivity" here.
"In the first place, what is the difference between radiation and radioactivity?"
"What is a sievert?"
On these pages, we can solve such questions.

―Radioactivity―
The words "radioactivity" and "radiation" can often be heard on the news.
"What is the difference?"
"In the first place, what is radioactivity?"
This page explains such a question.

―Unit of determination―
On this page, I explain about the unit of measuring radiation and radioactivity.
I think we hear the words "sievert" and "becquere" often.
These are both units of radiation and radioactivity.

―Mechanism of the radiation―
On this page, I explain about the mechanism of the radiation.

―Isotope―
Maybe you can't understand well if you hear the word "isotope".
When you hear the news that
"Cesium-134 of ○○ becquerels and cesium-137 of □□ becquerels were detected".
I think that you may wonder that
"It is the same cesium, but why are the numbers different?"
This page holds the answer to that question.

―Half-life―
On this page, I explain the half-life that is part of radioactive material's nature.
Maybe, this word is unfamiliar to you,
but this is very important
because this property become a criterion to strength of the radioactivity becomes harf.