The term is, to an extent, ambiguous. It can refer either to the "tallit katan", an item that can be worn over or under clothing and commonly referred to as "tzitzit", or to the "tallit gadol", a Jewish prayer shawl worn over the outer clothes during the morning prayers (Shacharit) and worn during all prayers on Yom Kippur. The term "tallit" alone, usually refers to the tallit gadol.
There are different traditions regarding the age from which a tallit gadol is used, even within Orthodox Judaism. In some communities, it is first worn from bar mitzvah, (though the tallit katan is worn from pre-school age). In many Ashkenazi circles, a tallit gadol is worn only from marriage, and in some communities it may be customarily presented to a groom before marriage as a wedding present or even as part of a dowry.
The Bible does not command wearing of a unique prayer shawl or tallit. Instead, it presumes that people wore a garment of some type to cover themselves and instructs the Children of Israel to attach fringes (ציצית tzitzit) to the corners of these (Numbers 15:38), repeating the commandment in terms that they should "make thee twisted cords upon the four corners of thy covering, wherewith thou coverest thyself" (Deuteronomy 22:12). These passages do not specify tying particular types or numbers of knots in the fringes. The exact customs regarding the tying of the tzitzit and the format of the tallit are of post-biblical, rabbinic origin and, though the Talmud discusses these matters, slightly different traditions have developed in different communities. However the Bible is specific as to the purpose of these tzitzit, stating that "it shall be unto you for a fringe, that ye may look upon it, and remember all the commandments of the LORD, and do them; and that ye go not about after your own heart and your own eyes, after which ye use to go astray; that ye may remember and do all My commandments, and be holy unto your God".
Encyclopaedia Judaica describes the prayer shawl as "a rectangular mantle that looked like a blanket and was worn by men in ancient times". Also, it "is usually white and made either of wool, cotton, or silk".
Traditionally the tallit is made of wool or linen, based on an understanding that reference to a "garment" in the bible in connection with a mitzvah refers specifically to wool and linen garments. Though other materials are sometimes used, the debate has not reached a conclusion, and many, especially among the orthodox, prefer wool which is accepted by all authorities. There is also debate about mixed wool and linen tallit, since the bible forbids klayim (shatnez) - "intertying" wool and linen together, with the two exceptions being garments of kohanim and tzitzit. Concerning tzitzit, chazal (the sages) permit using wool and linen strings in tandem only when genuine tekhelet (see below) is available, whereas kabbalist sources take it a step further by encouraging its practice.