Articles come before nouns and help you understand what exactly we're talking about. They are also declined (change their form) to give you information about the noun's...
In English, the definite article is "the":
Definite articles are used when talking about something specific.
⇒ The noun is unique. You can say exactly what you are talking about.
Definite articles are used when talking about something generally known or already specifically mentioned.
In English, "a" and "an" are indefinite articles:
Indefinite articles are used when we talk about something unspecified.
⇒ The noun is not unique. It is one of many.
(There are many sticks. It could be any of them.)
In some instances, articles are combined with a preposition:
These pairs are always combined.
If you say it separately, everyone will still understand you, but there is a difference in meaning: the noun is really emphasized (like the English "this," which is a demonstrative pronoun).
When do you use der, die, and das? Everything about the topic of gender can be found here: Articles and Gender.
Guess articles right 75% of the time even when you haven't memorized it yet! Article Trick.
When do you use der / die / das, and when ein / eine? ⇒ Definite and Indefinite Articles
Sometimes we don't use any article: The Zero Article.
Using an article to negate a noun: „Kein“.