Hamavdil is a piyut sung following Havdala on Saturday evening. The words were composed by Isaac Ibn Chayat, and it was originally part of the N'ila service on Yom Kippur. This is a traditional Yekkish melody, which I learned from Daniel and Chavivah Van Der Plaat, though I have arranged the version as transcribed by Neil Levin in his Z'mirot Anthology.