As far as the meter goes, we can start by counting out the number of syllables. The first and third lines have sixteen syllables each. That makes eight pairs of syllables. The emphasis in these pairs is usually placed on the first syllable: (Once u/pon a /midnight/ dreary). This kind of syllable pair is called a trochee. Since there are eight syllable pairs in a line, we call it "octameter" (octo- standing for eight, as in octopus). So the meter is called trochaic octameter.