There are two kinds of articles in English:
There are also cases when we don't put any article. We also call it "zero" article.
Which article to use, depends on the noun and on the context. It is important whether the noun is countable or uncountable, singular or plural, whether it indicates a class of objects or a specific object. Furthermore, there are rules for set expressions and names.
Type of a noun and articles
one of many
Water, oil and milk are liquids.
A pilot flies a plane.
I've got apples and bananas.
I left the phone in the car.
The flowers you sent are beautiful.
The coffee is cold.