Arabic is a cursive script in which letters within a word are normally joined, and it is one of the few scripts in the world for which cursive writing is not a style but the ordinary and only form. Print, handwriting and inscription all connect the letters. There is no equivalent of the roman distinction between separated capitals and joined script.
Six letters break that rule. Alif, dal, dhal, ra, zay and waw connect to the letter before them but not to the letter after, so each one closes the connected group it stands in. The next letter must begin a new group in its initial or isolated form. This is why a single Arabic word may appear on the page as two, three or four separate clusters, and why counting clusters is no guide to counting words.
The behaviour is inherited rather than designed. The Nabataean cursive from which Arabic descends had already begun to link some letters and not others, and Arabic fixed the pattern that had emerged. Its consequences run through the script. Because the six non-joiners produce a natural gap, calligraphers use them to control the density and rhythm of a line, and the classical hands have rules about how far a preceding letter may be stretched to reach one. Typesetting and, later, digital text encoding had to model the same behaviour: rendering an Arabic string means choosing among the isolated, initial, medial and final form of each letter according to its neighbours, which is why an alphabet chart of isolated forms represents the letters accurately and represents Arabic writing hardly at all.