On Friday evening, we sanctified the Shabbat with the kiddush ritual; now, as we take leave of it after a night and day of divine rest, we once again pronounce the holiness of the day over a cup of wine.
The Havdalah ("Separation") ceremony is a multi-sensory ritual employing our faculties of speech and hearing, sight, smell and taste to define the boundaries that G-d set in creation "between the sacred and the everyday."
Paradoxically, this act of separation is what connects Shabbat with the rest of the week. When the boundaries between the holy and the ordinary are blurred, the holy is no longer holy and the ordinary is left with nothing to uplift it. By defining the separation of Shabbat from the workday week, the relationship between the two is also established -- a relationship in which Shabbat imparts its transcendent vision to the rest of the week, and the six days of daily life feed into, and are sublimated within, the sanctity of Shabbat.