Centromere

The centromere is something like a center of a chromosome, that binds together the sister chromatids when necessary (i.e. when there are two chromatids - during cell replication).
It plays a major role during the cell replication, because the chromatids are torn away from the centromere regions. In the centromere, there usually aren't any functional DNA sequences, but sometimes there are.
Kinetochore is a protein in the centromere area that does the main binding. Some organisms have it along all the chromozome, so in fact there is nothing like a centromere.
Errors in centromere creation may lead to genetic diseases - e.g. Down syndrome.