A, an and the are called the articles. A/an is called the indefinite article; the is called the definite article.
We use the before a noun when our listener/reader knows (or can work out) which particular person(s) or thing(s) we are talking about. In other cases, we use a/an, some/any or no article.
I have been to the doctor. (You know which one: my doctor.)
A doctor must like people. (= any doctor)
The conventional name for the English determiner a or an. We can use a/an to talk about one particular person or thing, when the listener does not which one is meant, or when it does not matter which one.
John is going out with a German girl.
Could you give me a pen?
They have a big house in the city.
We can also use a/an to talk about any one member of a class.
A spider has eight legs.
A doctor must like people.