One may not eat or drink—even a sip of water—from “the time of Kiddush” on the eve of Shabbos and Yom Tov until after Kiddush. There is a discussion among poskim regarding what is considered “the time of Kiddush”; the consensus is that it begins at the time of shkiah (sunset), when the period of time called bein hashmashos begins. However, in case one accepts Shabbos early, prior to sunset (but after the time of plag haminchah) by davening Maariv early, or if people are sitting and eating and drinking together before sunset and they then say, “Bo’u u'nekadesh Kiddush hayom” (“Come, let us go and make Kiddush”), in either of these two cases from that moment onward eating and drinking are not permitted without making Kiddush.
Another issue addressed by poskim is to what extent a woman’s candle lighting constitutes the acceptance of Shabbos, and consequently, whether she may eat or drink after she lights. The halachah is that in case a woman is hungry or thirsty when it is time to light, she can make a stipulation that she is not accepting Shabbos by lighting candles, and then she may eat or drink. She must however accept Shabbos within ten minutes of lighting candles. On Yom Tov when she says Shecheyanu when lighting, such a stipulation is not effective.