Centromeres in higher eukaryotes usually consist of long repeated base sequences. In humans these repeated sequences are known as Alpha satellites. In eukaryotes such as S.cerevisiae the centromere consists of a much smaller sequence (126 bp) and is not a repeat.
Centromeres do not necessarily have to be in the centre of the chromosome, and chromosomes be classified according to position of the centromere. Metacentric chromosomes have their centromere in the centre of the chromosome, submetacentric have centromeres slightly off centre. Arcocentric chromosomes have long arms (q) and short arms (p) like submetacentric chromosomes, however the centromere is slightly higher leaving longer long arms and shorter short arms. Telocentric chromosomes have their centromere at the top of the chromosome.