Definite and Indeinite Articles - معرفة ونكرة - (marifah wa nakirah)

In English:
A book - this is called indefinite article
The book - this is called definite article
In Arabic:
Most of the indefinite articles are indicated by a tanween. for example, كِتَابٌ - kitabun. This means 'a book'. the taween at the end indicates that it is an indefinite article.

To make this a definite article, we just need to add al (ال) in the begining and chnage the tanween to single harakah.
كِتَابٌ = kitabun => A book
ال + كِتَابٌ = الْكِتَابُ = alKitabu => The book
[al + kitabun = alKitabu => The book]