Arabic is written from right to left, like other Semitic scripts, and consists of 17 characters, which, with the addition of dots placed above or below certain of them, provide the 28 letters of the Arabic alphabet.
Short vowels are not included in the alphabet, being indicated by signs placed above or below the consonant or long vowel that they follow. Certain characters may be joined to their neighbors, others to the preceding one only, and others to the succeeding one only. The written letters undergo a slight external change according to their position within a word. When they stand alone or occur at the end of a word, they ordinarily terminate in a bold stroke; when they appear in the middle of a word, they are ordinarily joined to the letter following by a small, upward curved stroke. With the exception of six letters, which can be joined only to the preceding ones, the initial and medial letters are much abbreviated, while only to the preceding ones, the initial and medial letters are much abbreviated, while the final form consists of the initial form with a triumphant flourish. The essential part of the characters, however, remains unchanged.
These features, as well as the fact that there are no capital forms of letters, give the Arabic script its particular character. A line of Arabic suggests an urgent progress of the characters from right to left. The nice balance between the vertical shafts above and the open curves below the middle register induces a sense of harmony. The peculiarity that certain letters cannot be joined to their neighbors provides articulation. For writing, the Arabic calligrapher employs a reed pen (qalam) with the working point cut on an angle. This feature produces a thick downstroke and a thin upstroke with an infinity of gradation in between. The line traced by a skilled calligrapher is a true marvel of fluidity and sensitive inflection, communicating the very action of the master's hand.
Arabic calligraphy, thus, is the art of beautiful or elegant handwriting as exhibited by the correct formation of characters, the ordering of the various parts, and harmony of proportions.
In the Islamic world, calligraphy has traditionally been held in high regard. The high esteem accorded to the copying of the Quran, and the aesthetic energy that was devoted to it, raised Arabic calligraphy to the status of an art. Arabic calligraphy, unlike that of most cultures, influenced the style of monumental inscription. It is revered as highly as painting.
Source: Encyclopedia Britannica