The definite article in Arabic is used to make a noun or an adjective definite. In English, "a dog" is made indefinite by the use of the article "a" and it is made definite with the article "the" as in "the dog". The word "dog" is not effected. Arabic uses the definite article أَلْ which it attaches to the noun or adjective itself. In English this is treated as two seperate words but in Arabic it is treated as one word. When the أَلْ is attached, the noun or adjective also loses its nunation which if you recall is the doubled vowel markers at the end of an indefinite noun or adjective. The doubled vowel markers at the end become single vowel markers.
The hamza of أَلْ which sits on top of the 'alif' is a special hamza. If the definite noun with أَلْ occurs at the very start of a sentence then the hamza over the 'alif' acts as a normal hamza and the vowel associated with the hamza is pronounced. However, if the definite noun occurs later in the sentence then the hamza becomes what is known as hamzatu l-wasl (هَمْزَةُ الْوَصْلِ). It is written as
ٱ
The hamzatu l-wasl tells the reader that the fatɧa above the 'alif' in أَلْ is NOT to be pronounced. Instead the ending vowel of the previous word is pronounced instead. For example:(The new book is here)
is pronounced as
and NOT as
While the hamzatu l-wasl may seem like a trivial nuance it is actually quite important if you want to speak proper Arabic and not sound like you're just starting out learning Arabic. Just keep in mind that you don't want to have two short vowels next to each other. The hamzatu l-wasl avoids this problem.
When the word to which the article أَلْ is attached begins with certain letters called "sun letters" (أَلْحُروفُ الشَّمْسِيَّةُ) , the sukūn over the ل is dropped and the ل is not pronounced. Additionally, the "sun letter" following the ل has a shadda placed over it, doubling its sound. Thus , for example, أَلْحُروفُ ٱلشَّمْسِيَّةُ is pronounced as al-ɧurūfu-sh shamsīya (notice the hamzatu l-wasl). There are 14 "sun letters" where this occurs.
From a phonetic point of view, all the above sun letters are pronounced in the same area of the mouth as the ل is, (ie. at or behind the upper teeth). When you think about it, it makes sense since we all like to take the path of least resistance when speaking. For example in English, we can say "What is up?" or we can say "Wazuuup?". While proper spoken English does not allow for such sloppy enunciation, Arabic has created rules for proper pronunciation with regards to the use of أَلْ based on ease of pronunciation.