Waw is the twenty-seventh letter of the modern Arabic order and the sixth of the older abjad order, from which it takes the numeral value six. It is written as a small loop with a tail curving down and back below the line, and it belongs to the group of six letters that join only to the right, so it ends the connected group it stands in and the next letter starts a fresh one.
Waw is a moon letter, so the lam of the definite article is pronounced before it.
Its name descends from the Northwest Semitic waw, the cognate of Hebrew vav and Syriac waw, whose sense was reconstructed as a nail or a hook, and it holds the numeral value six that Hebrew vav holds. The shape of the modern letter, a loop and a tail, has drifted a long way from anything hook shaped, and the reconstruction belongs to the older linear script rather than to the Arabic form.
Waw is one of the letters of the throat in the older classifications only marginally, and its main interest is grammatical rather than phonetic: it is one of two letters in the script that serve as consonant, as long vowel and as an independent word. That triple duty makes it one of the commonest marks on an Arabic page, and it is one of the reasons a page of Arabic is broken into so many connected groups, since every waw ends one.