1. ‘a’ / ‘an’ is called the indefinite article, and we use it to introduce new information

I read an interesting report last week.

2. ‘the’ is called the definite article, and we use it when the listener knows which person or thing we are talking about.

I read the report you gave me.

3. ‘no article’ is the noun by itself. Without an article. We use no article when we want to speak generally.

Reports are sent out four times a year.