On Yom Kippur, there is a Torah Prohibition to perform Melacha. Unlike other holidays, the restriction on Melacha on Yom Kippur is identical to Shabbat. Therefore, it is prohibited to carry without an Eruv, to cook and to handle any items defined as Muksa. The penalty for intentionally performing Melacha on Yom Kippur is Karet (early death from Heaven), whereas one who does Melacha on Shabbat is liable to receive Sekila (death by stoning).
In addition to the prohibition of Melacha, there is a positive commandment to refrain from the five categories of bodily pleasures: eating and drinking, bathing, anointing, marital relations, and wearing leather shoes. According to the Rambam, all five categories are prohibited M’Doraita (from Torah law). Nevertheless, the severe punishment of Karet is only for eating and drinking. One who violates the other categories is not liable Karet, but he has violated a Misva from the Torah. Most other early authorities understand that the restrictions other than eating and drinking are prohibited M’Drabanan (Rabbinic law). All the prohibitions of Yom Kippur apply equally at night and the day.
The prohibition of eating includes all types of foods, and the prohibition to drink includes all types of beverages. The punishment of Karet only applies if one consumed the minimal measure of a "plump date," equivalent to 36 grams within a short period. This measure is irrespective of whether the one eating is a giant or a midget. With regard to drinking, the minimum measure to be liable Karet is "M’loh Lugmo" (a cheek-full). This measure is relative to the size of the mouth of the one drinking. In an average person, this is equivalent to 1.5 ounces. Different types of foods are calculated together to reach the minimal measure, and so are different types of beverages. However, even if one ate or drank less than the minimal measure, he has still violated a Torah prohibition. He is just not liable the penalty of Karet.
The requisite measure must be consumed within 10 minutes from the beginning of the first bite until the last bite. Therefore, if one ate a small amount and after five minutes ate another small amount, such that he ate a total of more than 36 grams within ten minutes, he is liable. With regard to drinking, the minimum measure must be swallowed in a very short time of under 3 seconds (the amount of time it takes to drink a Revi’it). This results in a leniency that if a person drank just under 1.5 ounces and then waited a few seconds and took another sip, he is not liable Karet, but of course, he has still violated a Torah prohibition.