In ancient times, most Jews used to wear a Tallit over their cloths during the day, but in time, the Tallit was no longer worn as an outer garment but an inner one. Some think this change took place after Israel was exiled from the land because the Tallit wearer, clearly marked as a Jew, would have been subject to persecution or discrimination.
Very religious Jews still wear the "innerwear" Tallit, with only the Tzitzit (the fringes of the Tallit) hangin out of their cloths, so they could be seen. In this way they fulfill the commandment of Numbers 15:39, which requires the wearer to "look upon" the fringes. This inner Tallit is called the Tallit Katan or Small Tallit.
The Tallit Katan should be at least 16 x 16 inches in the front and in the back, and it must have four corners. The Tallit Katan is usually referred to simply as Tzitzit.