A chromosome translocation occurs when a piece of one chromosome breaks off and sticks to another chromosome. There are two types of chromosome translocations - reciprocal and Robertsonian (named for the Dr. that first described it).
A reciprocal translocation occurs when pieces of two different chromosomes exchange places. A Robertsonian translocation occurs when a whole chromosome attaches to another chromosome. In Down syndrome, most translocations are Robertsonian translocations involving chromosome 21.